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)

[changeset-bot]

🦋 Changeset detected

Latest commit: f9e2f71

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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