Fix OpenAI streams with empty tool_calls arrays

[lc4t]

Summary

• preserve OpenAI-compatible SSE chunks whose deltas contain an empty tool_calls array
• add regression coverage for content and terminal chunks with tool_calls: []
• document the user-visible fix in the pending release notes

Root cause

The OpenAI SSE interceptor treated any present delta.tool_calls value as a handled tool-call delta. Because empty arrays are truthy in JavaScript, providers that include tool_calls: [] on ordinary content or terminal chunks caused those chunks to be suppressed. Pi then received [DONE] without the upstream finish_reason: "stop" chunk and reported Stream ended without finish_reason.

Changes

• require delta.tool_calls to contain at least one entry before entering the tool-call buffering path
• verify that content, finish_reason: "stop", and [DONE] pass through when tool_calls is empty
• retain the existing behavior for non-empty, repeated, parallel, and shifted-index tool calls

Testing

• bun test across the OpenAI interceptor SSE, relay, hint, response-repair, and validation suites: 43 passed, 0 failed
• bun run typecheck:shared
• replayed an OpenAI-compatible SSE stream through the interceptor and confirmed content, finish_reason: "stop", and [DONE] are all preserved
• git diff --check

Validation note

bun run typecheck:all could not complete because the v0.11.2 checkout does not contain the root tsconfig.base.json referenced by several package configs. The affected packages/shared typecheck passes.

Fixes #995