Studio: preserve durations across reasoning blocks (#7520)

* Studio: preserve durations across reasoning blocks

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Studio: keep a reasoning group's timer running when it reopens

A rendered reasoning group can be closed and then reopened: parseAssistantContent
coalesces adjacent reasoning parts, so a provider that emits each block as a
complete <think>...</think> chunk lands several blocks in one group. The tracker
wrote a group's duration once and never revisited it, so such a group froze at
its first close and displayed 0 seconds.

Measure from the first time an index becomes visible rather than from the last
startGroup, and reopen a closed group while its reasoning text is still growing.
Gating on growth is what stops the timer running on into the answer. A duration
supplied by the server is now recorded as authoritative so local timing cannot
overwrite it.

Also fill indices that a single delta skips. startGroup(n) could jump past
earlier indices and leave array holes, which JSON.stringify persists as null; a
skipped group became visible and closed inside the same chunk, so it gets a
measured zero instead.

Test discovery now globs tests/, so a second test file cannot be silently
skipped by CI, and tsconfig.test.json puts tests/ under typecheck for the first
time.

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: danielhanchen <danielhanchen@gmail.com>

.github/workflows/studio-frontend-ci.yml

@@ -133,6 +133,9 @@ jobs:
       - name: Typecheck
         run: npm run typecheck
 
+      - name: Unit tests
+        run: npm test
+
       - name: Build
         run: npm run build