fix(antigravity): parse unterminated SSE events

[outlier27-cell]

Problem

Antigravity parsed data: fields one line at a time during normal SSE reads, but treated an unterminated EOF buffer as one line. A valid multi-line event without its final blank delimiter therefore failed JSON parsing.

Closes #784

Value

The provider accepts a valid final multi-line SSE event when a transport closes without its optional event delimiter.

Approach

• Reuse one data: line collector during normal reads and EOF processing.
• Add a stream-level regression with two data lines and no trailing blank line.

Validation

• node --test --experimental-strip-types --test-name-pattern "unterminated multi-line SSE event" tests/extensions/ai-providers/antigravity.test.ts
• bun run check
• bun run test (Windows baseline failures outside this change: file-lock/symlink-permission and existing TUI assertions)

Impact

No API changes. Normal delimited SSE handling, timeouts, and cancellation behavior are unchanged.

[outlier27-cell]

@testikun @tt-a1i CI is green and the scoped regression coverage is complete. Please review when available.

[tt-a1i]

Thanks! I couldn't reproduce #784 on current main: with the provider change reverted, the new test streamAntigravity parses an unterminated multi-line SSE event at EOF still passes, because the read loop already consumes every complete line and only the final line reaches the EOF tail. Could you add a regression that fails on main (e.g. a chunking/\r\n case that really leaves several data: lines in the tail), or close this if the bug isn't reachable?