fix: stabilize claude streaming event parsing

[rexdotsh]

Summary

• switch Claude SSE rewriting to full event-boundary parsing modeled after pi instead of line-by-line rewriting
• stream transformed Claude responses from a push loop so long sparse thinking gaps do not stall client output
• add a regression test for fragmented multiline Claude SSE events

Testing

• bun test tests/providers/proxy-contract.test.ts
• bun typecheck