fix: surface duplicate AI provider hostname as warning, not error - #28208

Two live ai_providers rows can share the same base_url hostname. This
is an intended configuration (AIGOV-97): the direct routing path
(/api/v2/aibridge/{name}/...) routes by provider name and fully
supports it. Only the MITM proxy path cannot disambiguate, because
its nameByHost map is 1:1.

classifyProviderRow marked the second provider as ProviderStatusError,
which surfaced as a misleading log warning and a status=error
Prometheus label. The provider is not actually broken; it is fully
routable via the direct path. The error was only visible in server
logs and Prometheus, not in the API or UI.

This change:

- Extracts BaseURLHostname into coderd/aibridged and eliminates the
  three duplicate implementations (proxy classifier, chatprovider,
  and the API layer now uses it for status checks).
- Downgrades the proxy classifier from ProviderStatusError to a new
  ProviderStatusProxyExcluded status. The provider is not broken;
  it is just not reachable via the MITM proxy path.
- Adds Status *AIProviderStatus to codersdk.AIProvider with
  Warnings []string, matching the WorkspaceProxy precedent.
- Populates warnings on create, update, GET, and list when a
  provider's hostname collides with another enabled provider.
- Adds an integration test proving that a duplicate provider is
  routable via the direct path but not via the proxy path.

Refs AIGOV-596

Add a warning badge with tooltip to ProviderRow when the API returns
status.warnings for a hostname collision. The badge is keyboard-
focusable and includes the full warning text in its aria-label.

Includes Storybook story with play function asserting the badge is
visible.

- Move proxy_excluded from aibridged provider_info help text to
  aibridgeproxyd provider_info help text; the aibridged metric never
  emits proxy_excluded, only the proxy classifier does
- Log GetAIProviders errors in aiProviderHostnameWarnings instead of
  silently swallowing them
- Update monitoring.md to enumerate proxy_excluded for the proxy
  provider_info metric

Addresses CRF-1, CRF-2, CRF-3, CRF-4, CRF-5 from deep-review round 1.

Update scripts/metricsdocgen/metrics with the corrected help text
for coder_ai_gateway_proxy_provider_info, then regenerate
docs/admin/integrations/prometheus.md.

Addresses CRF-5 from deep-review round 2.

- CRF-7: only warn the alphabetical loser, not the winner; the
  winner is reachable via the proxy
- CRF-8: rebrand warning message to AI Gateway and
  /api/v2/ai-gateway/ path
- CRF-9: eliminate N+1 query in list handler by building the
  hostname collision map once from the outer rows
- CRF-10: explicitly filter disabled and deleted peers in the
  collision map builder
- CRF-11: stop event propagation on warning badge so clicking it
  does not navigate to the edit page
- CRF-13: document proxy_excluded status in monitoring.md
- CRF-14: log proxy_excluded at Info level, not Warn

- CRF-16: BaseURLHostname docstring now says it IS the canonical
  normalization, not that it matches the classifier
- CRF-18: story play function asserts aria-label contains the
  collision hostname
- CRF-19: remove nested TooltipProvider so badge uses the app's
  default 100ms tooltip delay
- CRF-20: ProviderOutcome doc enumerates proxy-excluded rows
- CRF-21: restore already-logged-at-call-site invariant in
  ProviderOutcome doc
- CRF-22: add Linear URL to AIGOV-596 test comment

- CRF-24: buildHostnameCollisionMap preserves DB ORDER BY name ASC
  instead of re-sorting with Go byte order, matching the proxy
  classifier's winner selection on all collations
- CRF-25: add onKeyUp stopPropagation to warning badge so Space
  does not navigate the row
- CRF-26: warning message now says AI Gateway Proxy, not the
  umbrella AI Gateway
- CRF-27: add proxy_excluded row to providers.md status table

- CRF-28: story play function now tests keyboard activation (Enter
  and Space) on the warning badge to prevent regression of the
  stopPropagation fix
- CRF-29: aiProviderHostnameWarningFromMap docstring now says
  database order, not alphabetical
- CRF-30: monitoring.md now says database name sort order, not
  alphabetically first
- CRF-31: monitoring.md now advises excluding proxy_excluded from
  status-based alerts
- CRF-33: warning badge uses named stopPropagation function instead
  of three inline closures

- CRF-29: inline comment now says database order, not alphabetical
- CRF-32: declined; the API warning and classifier error serve
  different audiences (API consumer vs operator reading logs) and
  are maintained in separate packages with different imports.
  Extracting a shared string would couple the API layer to the
  proxy classifier's error format for no behavioral gain.

This reverts commit 9fdd080. The cleanup introduced regressions:
- CRF-37: warnings?.length can leak 0 in React JSX
- CRF-38: stopPropagation comment lost the three-handler invariant
- CRF-39: buildHostnameCollisionMap doc lost the ORDER BY tie
- CRF-22: AIGOV-596 reference lost its URL again

The panel noted the cleanup was unsolicited on a converging PR.

- CRF-40: rename allProviders to enabledProviders; the query
  filters disabled and soft-deleted rows
- CRF-42: story now tests mouse click on the warning badge
- CRF-43: add EmptyWarnings story to guard against the 0-leak
  JSX guard regression
- CRF-44: ProviderOutcome doc now says 503 sentinel, matching
  the codebase vocabulary
- CRF-45: providers.md now says database name sort order
  (ORDER BY name ASC), matching monitoring.md

CRF-43: the story asserts no bare "0" text appears in the row,
which fails if the JSX guard is reverted from
warnings && warnings.length > 0 to warnings?.length.

CRF-46: the block doc, mid-body invariant, and assert message
already carry the claim; the loop-header comment restated it
a third time.

- Inline winner := names[0] into the Sprintf call
- Remove default: continue from the Reload switch; falling
  through already continues the loop

- Fix PromQL example: status!~"enabled|proxy_excluded" instead
  of repeating the same label matcher key
- Add tooltip hover assertion to warning badge story