fix(session): coerce legacy string tool-part input

[LovePlayCode]

Issue for this PR

Closes#44688

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

1.14 stored some tool-part state.input values as JSON strings. 1.18 encodes messages with Schema.Record, so GET /session/:id/message 400s the whole transcript.

Hydrate now parses those strings (invalid JSON / non-objects become {}). A data migration rewrites the stored part rows the same way.

How did you verify your code works?

• bun typecheck in packages/core and packages/opencode
• bun test test/session/message-v2.test.ts in packages/opencode (hydrate + HTTP encode)
• bun test test/database-migration.test.ts in packages/core (SQL backfill)

Screenshots / recordings

If this is a UI change, please include a screenshot or recording.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

Made with Cursor

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found one potentially related PR:

PR #44535 - fix(session): stop creating phantom "unknown" tool parts on re-emitted deltas
#44535

This PR is related because it also addresses tool-part handling in sessions, though it focuses on preventing creation of phantom tool parts rather than coercing legacy string inputs.

The other PRs found (like #37541 and #41830) address broader session message encoding and structure, but are less directly related to the specific legacy string coercion issue that PR #44705 is fixing.