fix(sdk): validate exec status before yielding exit

[dipeshbabu]

Summary

Check the final RPC status before exposing a TypeScript exec stream's exit event. Previously a caller that stopped at exit could silently miss a subsequent transport failure, and events received after exit were accepted.

Related Issue

No issue required: localized correction of existing exec stream completion semantics, without changing public types.

Changes

• Retain the exit code until RPC completion while continuing to stream stdout and stderr immediately.
• Reject duplicate exit events and output received after exit.
• Add regressions for a late RPC failure and malformed event ordering.
• Document when the terminal event is emitted.

Testing

• mise run pre-commit passes. The full-repository attempt stopped on missing tooling; the commit hook was bypassed. The passing SDK checks below do not establish full repository CI success.
• All three regression cases fail on the original implementation.
• mise run --skip-tools sdk:ts:ci passes: protobuf lint/generation, Biome, type checks, build, and 144 tests.
• Line coverage: 91.73%, above the existing 80% threshold.
• Full mise run ci passes. Repository-wide validation remains blocked.
• E2E tests added/updated (not applicable to the client-side terminal-status correction).

Checklist

• Follows Conventional Commits.
• Commit is signed off (DCO).
• Architecture and SDK documentation updated.

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