fix(cursor): bound protobuf map entries

[outlier27-cell]

Problem

Cursor protobuf map decoding kept reading from the outer message after a map entry's declared length. A malformed entry could therefore consume later bytes as its value.

Closes #735

Value

Malformed catalog protobuf input now fails at the entry boundary instead of silently producing a map value from unrelated bytes.

Approach

• Decode each length-delimited map entry through its own bounded Reader.
• Add a regression test using the issue's truncated-entry shape and a valid control message.

Validation

• node --test --experimental-strip-types tests/extensions/ai-providers/cursor-protobuf.test.ts tests/extensions/ai-providers/cursor.test.ts (38 passing)
• bun run check
• bun run test was started; the unmodified full suite reproduced existing renderer-chain failures before it was stopped. Exact-head GitHub CI is tracked for the final cross-platform result.

Impact

No configuration or user-visible workflow changes. The Cursor protobuf decoder rejects malformed map entries more accurately.

[outlier27-cell]

@tt-a1i 请审核此 PR。#735 的 protobuf map entry 边界读取已在 07d5f23 修复；专项 38 项、�un run check 与 exact-head GitHub CI（Node、Windows、UI、Web E2E）均通过。当前仅等待审核。