docs(changelog): v0.43.0

[hung12ct]

What & why

CHANGELOG entry for v0.43.0, covering the four changes merged in #146–#149. Required before the tag is cut.

Changes

• ### Added — WithMaxParallelToolCalls, the ErrTimeout/DeadlineCause/TimedOut deadline-attribution trio, WithRequestInvariant/RequestViolationFunc.
• ### Changed (breaking defaults) — wave concurrency now defaults to 8 rather than unlimited. Results are unaffected (calls are delayed, not dropped), but wall-clock rises for turns that previously issued a wide fan-out at once. WithMaxParallelToolCalls(0) restores the old behaviour.
• ### Fixed — the mid-wave abort saving an unbalanced transcript and discarding completed results; the code-interpreter and video-generation timeout misattribution; the opaque SQL query-budget error.
• Version link reference appended.

Docs only — no code in this PR.

Testing

• Verified every file path cited in the entry exists, and every symbol named (WithMaxParallelToolCalls, ErrTimeout, DeadlineCause, TimedOut, WithRequestInvariant, RequestViolationFunc) resolves in the tree.
• Re-ran the full gates on the release content: gofmt -l . empty, make lint 0 issues, make build, make test, make test-race all clean.

Checklist

• Title is a Conventional Commit
• gofmt -l . prints nothing
• make lint is clean
• make build passes
• make test (and make test-race) passes
• Errors wrapped with a package prefix (n/a — docs only)
• CHANGELOG.md updated — this PR is the update; tag v0.43.0 follows on merge
• No changes to pkg/llm/, pkg/history/, or pkg/telemetry/