fix: route Playground ChatTab Send to the selected endpoint (#10592)

[diegosouzapw]

Closes #10592

Root cause

ChatTab.tsx (the "Send"-style chat UI in the main Playground) hardcoded every
request to POST /api/v1/chat/completions and never read configState.endpoint,
even though StudioConfigPane's "Endpoint" selector exposes 13 real API endpoints
(chat.completions, search, web.fetch, etc. via ENDPOINT_OPTIONS /
endpointToPath() in src/lib/playground/codeExport.ts). Search providers
(exa-search, tavily-search, serper-search) have no chat-model catalog entries,
so StudioConfigPane falls back to a free-text model field, letting a user type
exa-search/web and hit "Send" — which still POSTed to chat.completions and 404'd
with No active credentials for provider: exa-search, because chat credential
resolution has no notion of search-provider credentials.

Fix

ChatTab.tsx now resolves the request path from configState.endpoint via the
existing endpointToPath() helper, and for non-chat-shaped endpoints (currently
search and web.fetch) builds a query/url-based request body instead of the
messages[] shape, then renders the raw (JSON-pretty-printed when possible)
response instead of assuming an SSE chat-completions delta stream. The
chat.completions path (the default/most common case) is untouched — same
request body builder, same SSE decode loop.

New module: src/app/(dashboard)/dashboard/playground/components/tabs/chatTabEndpointRequest.ts
(kept ChatTab.tsx's diff minimal and gives the endpoint-routing logic its own
testable seam).

Regression test (TDD)

tests/unit/ui/playground-chat-tab-search-endpoint-10592.test.tsx — renders
ChatTab with configState.endpoint: "search", types a query, clicks Send, and
asserts the captured fetch() URL is /api/v1/search (not
/api/v1/chat/completions).

• RED on origin/release/v3.8.50 (confirmed in the analysis pass and re-confirmed
  on this fresh worktree before the fix): AssertionError: expected '/api/v1/chat/completions' to be '/api/v1/search'.
• GREEN after the fix: npx vitest run --config vitest.config.ts tests/unit/ui/playground-chat-tab-search-endpoint-10592.test.tsx → 1 passed.

Gates run

• npx vitest run --config vitest.config.ts tests/unit/ui/playground-chat-tab-search-endpoint-10592.test.tsx → pass
• npm run typecheck:core → exit 0
• npx eslint --suppressions-location config/quality/eslint-suppressions.json <changed files> → clean
• node scripts/check/check-file-size.mjs → OK
• node scripts/check/check-complexity.mjs → OK (2573 violations vs baseline 2774)
• node scripts/check/check-cognitive-complexity.mjs → OK (1157 violations vs baseline 1223)
• node scripts/check/check-changelog-integrity.mjs → OK

Scope note

CompareTab.tsx has an independent hardcoded fetch(${configState.baseUrl}/v1/chat/completions, ...) call with the same class of issue, but it's a separate component/test surface — out of scope for this fix and left for a follow-up issue rather than widening this PR's diff.