fix(ai): preserve provider-defined responses item ids

[rekram1-node]

Summary

• replace item-type-specific Responses ID allowlists with the exact Codex outbound rule: the first underscore must have a nonempty prefix and suffix
• preserve provider-issued message, reasoning, function-call, and hosted-tool IDs regardless of semantic prefix, length, or characters; omit malformed IDs without discarding replayable reasoning
• add a Bedrock Mantle regression using its recorded msg_95d4d0af4350432a reasoning item ID, plus long opaque provider IDs, symbolic IDs, malformed prefixes, and ID-less reasoning coverage

Verification

• bun test test/partial-json.test.ts test/tool-stream.test.ts test/provider/openai-responses.test.ts test/provider/openai-compatible-responses.test.ts test/provider/bedrock-mantle.test.ts test/provider/openai-responses-images.recorded.test.ts test/provider/openai-responses-websocket.recorded.test.ts test/provider/openai-chat.test.ts test/provider/anthropic-messages.test.ts test/provider/bedrock-converse.test.ts test/provider/xai-responses.test.ts test/response.test.ts test/tool-runtime.test.ts --timeout 30000 --only-failures (390 passing)
• RECORDED_PREFIX=openai-responses bun test test/provider/golden.recorded.test.ts --timeout 30000 --only-failures (4 passing)
• bun typecheck from packages/ai
• bunx prettier --check packages/ai/src/protocols/open-responses.ts packages/ai/src/protocols/openai-responses.ts packages/ai/test/provider/openai-responses.test.ts packages/ai/test/provider/openai-compatible-responses.test.ts packages/ai/test/provider/bedrock-mantle.test.ts
• Full-package test run also encounters existing executor, Anthropic, and OpenRouter failures; representative failures were reproduced unchanged in a pristine v2 worktree.