feat: Improve SQL editor completions for DuckDB functions and columns - #665

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@packages/sql-editor/src/codemirror/extensions/completion.ts`:
- Around line 84-97: The suppression logic only inspects the current line via
lineAt/completionContext.pos, so multi-line clauses like "FROM\n  ord|" slip
through; change how textBeforeCursor is computed to look back a small window in
the document instead of only the current line: use completionContext.state.doc
and completionContext.pos to compute a start index (e.g. Math.max(0, pos - N)
with N ~100–300) and slice the document from that start to pos to produce
textBeforeCursor, then run the existing regex checks against that window (keep
the same regexes and return null behavior).
- Around line 29-57: The current logic only uses applyQuotedIdentifier when
needsIdentifierQuotes(...) is true, which misses reserved words like "select";
make createColumnCompletion always set apply to the custom apply function
(remove the conditional), and inside applyQuotedIdentifier compute const escaped
= escapeId(column.name) and use that for insertCompletionText; determine whether
to back up over an existing leading quote by checking whether
escaped.startsWith('"') (if true, and the char before `from` is '"' then set
replaceFrom = from - 1, otherwise leave replaceFrom = from), then call
insertCompletionText(view.state, escaped, replaceFrom, to). Reference:
createColumnCompletion, applyQuotedIdentifier, needsIdentifierQuotes, escapeId,
insertCompletionText.

In `@packages/sql-editor/src/codemirror/utils/schema-converter.ts`:
- Around line 42-43: The current logic in schema-converter.ts returns
existing.children as a NamespaceRecord whenever an object has a children
property (the block around the existing variable), which wrongly treats table
nodes (where children is a Completion[]) as namespace nodes; update the guard in
the code paths that return or merge existing.children (the if that checks
existing && !Array.isArray(existing) && 'children' in existing and the similar
block around lines 46–50) to verify children is an object map (e.g., not an
array and typeof children === 'object' && children !== null) before casting to
NamespaceRecord, and if children is an array preserve/merge it as completions
instead of treating it as a NamespaceRecord.

---

Outside diff comments:
In `@packages/sql-editor/src/SqlEditorSlice.tsx`:
- Around line 263-283: When reselecting after deleting a tab, guard against
deletingOpenIndex being -1 by clamping it into the valid range for
config.openTabs before using it: compute a safeIndex = deletingOpenIndex < 0 ?
Math.min(0, newOpenTabs.length - 1) (or simply use Math.max(0,
Math.min(deletingOpenIndex, newOpenTabs.length - 1))) and use that to pick
newSelectedId from newOpenTabs; if newOpenTabs[newIndex] is undefined (or
newOpenTabs is empty) fall back to picking a valid id from remainingQueries
(e.g., remainingQueries[0]?.id) or set selectedQueryId to null, and update
assignment sites that use newSelectedId/selectedQueryId accordingly (references:
deletingOpenIndex, config.openTabs/newOpenTabs, remainingQueries/config.queries,
newSelectedId).