fix: keep derived session titles from splitting emoji surrogate pairs

[soumojit-D48]

Summary

Derived session titles could end in a lone surrogate when the 50-character cap landed inside an emoji, storing malformed Unicode (visible as a replacement character in the session list). This PR backs off one unit when the cut would split a surrogate pair, so titles stay well-formed while keeping the existing length limit.

Closes #980

Changes

• packages/trueforge/src/apis/turns.ts: route deriveSessionTitle through a new truncateSessionTitle helper that drops a trailing high surrogate after capping.
• packages/trueforge/tests/unit/apis/turns.test.ts: new deriveSessionTitle block (cap behavior, short/empty inputs, intact pair inside the cap, split-pair back-off asserting the exact result).
• .changeset/session-title-surrogate-boundary.md: patch bump for @truefoundry/trueforge.

How was this tested?

• pnpm --filter @truefoundry/trueforge test -- tests/unit/apis/turns.test.ts: 14/14 pass (9 pre-existing + 5 new).
• pnpm --filter @truefoundry/trueforge typecheck: exit 0 (src + tests projects).
• eslint on the changed source file: clean (test files are eslint-ignored per repo config).
• prettier --check on all changed/added files: pass.
• Full-workspace pnpm build, pnpm test, and pnpm smoke were NOT run in this environment (Windows); CI covers the rest.

Checklist

• I have read the contributing guidelines
• pnpm build, pnpm test, pnpm typecheck, pnpm lint:ci, and pnpm format:check pass locally (package-level test/typecheck/eslint/prettier pass; full workspace gates pending CI — see above)
• Tests added/updated where it makes sense
• No hand-edits to generated code (packages/trueforge-sdk, python/trueforge_sdk, .github/fern/openapi/openapi.json, docs/openapi.json) — fork PRs omit SDK regen; maintainers regenerate after merge
• Docs / .env.example updated if configuration or behavior changed (not needed: no config or operator-visible behavior change beyond the bug fix)

---

Note

Low Risk
Localized first-turn title derivation and display formatting; no auth, API contract, or persistence schema changes beyond avoiding malformed or blank titles.

Overview
Fixes derived session titles so the 50-character cap no longer cuts through emoji or other multi-code-unit graphemes (surrogate pairs, skin tones, ZWJ sequences).

deriveSessionTitle now caps via a new truncateSessionTitle helper that walks Intl.Segmenter grapheme clusters and omits any cluster that would exceed the limit, instead of using a raw slice. If nothing fits—including when a single cluster is longer than the cap—the function returns undefined so the session store does not persist an empty title on first write.

Unit tests cover length limits, empty input, and several Unicode edge cases; @truefoundry/trueforge gets a patch changeset.

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[changeset-bot]

🦋 Changeset detected

Latest commit: c6acc41

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@truefoundry/trueforge	Patch

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[thesujai]

Thanks @soumojit-D48, just one change and this will be good to merge

[soumojit-D48]

Done — thanks for catching that, the surrogate back-off would indeed have mangled 👋🏽 into 👋.
@thesujai

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