feat: enable Coder Agents for organization members (#28186) - #28688

Batch backport of ibetitsmike's PRs that merged into `main` after the
`release/2.37` branch was cut (merge-base `c275327fb72`, 2026-08-24).

## Included (9 cherry-picks, in order)

| PR | Title |
|----|-------|
| #28470 | fix(coderd/x): surface Gemini malformed-function-call stream
deaths as retryable errors |
| #28471 | fix: apply MCP server selection when editing a chat message |
| #28460 | refactor: consolidate viewport hooks and remove
defineProperty matchMedia stub |
| #28476 | feat(coderd/x/chatd/chattool): improve find_tools relevance
and model guidance |
| #28400 | fix(coderd/x/chatd/mcpclient): enforce MCP connect budget and
unblock session cleanup |
| #28589 | fix(coderd/x/chatd): truncate overlong generated chat titles
instead of rejecting them |
| #28496 | feat: mount chat API routes under /api/v2 |
| #28497 | feat: promote codersdk chat API methods to Client |
| #28498 | feat(site): use /api/v2 chat API paths |

All picks applied cleanly with `git cherry-pick -x` onto `release/2.37`.

## Excluded and follow-up backports

The remaining post-cut PRs are cherry-picked onto this PR as a GitHub
stack (#28692) that merges bottom-up, so the dependency order is
enforced by the PR bases:

| Stack order | Original PR | Backport PR | Notes |
|-------------|-------------|-------------|-------|
| 1 | #28462 (repair failing Storybook stories) | #28687 | One fixture
adaptation: 2.37's `OrganizationModelsContextValue` has no
`organizations` field |
| 2 | #28186 (enable Coder Agents for organization members) | #28688 |
One story-assertion conflict resolved to the picked regex; ships no
migration by design |
| 3 | #28593 (allow sharing MCP servers with users and groups) | #28686
| Clean pick on top of #28688, plus the 1-line test fix #28657 (expect
v2 MCP ACL path) |
| 4 | #28659 (point MCP server ACL msw handlers at v2 paths) | #28689 |
Clean pick on top of #28686 |

Intentionally skipped: #28587 (restore agents-access cleanup migration).
Release branches cannot take a migration unless its sequence number
matches main with no gap.

## Validation

- `go build ./...` and test-binary compilation for `./coderd/...
./codersdk/... ./enterprise/... ./site` pass
- `pnpm check` and `pnpm lint:types` pass
- Targeted Vitest on all touched test files: unit 281/281 pass;
Storybook 180/181 pass. The single failure (`AgentChatPage.stories.tsx`
> Queued For Capacity After Polling) reproduces identically on the
untouched `release/2.37` baseline, so it is pre-existing and unrelated.

> Xum acted on Mike's behalf.
<!-- xum-attribution: model=claude-fable-5 thinking=high -->