fix(ai): merge Bedrock tool result messages - #41780

What

Merge adjacent canonical user and tool-result content when lowering requests for Bedrock Converse. This keeps every result for a parallel assistant tool-use response in the single immediately following Bedrock user message required by the provider.

Closes #41445

Before / After

Before: One assistant response with two parallel tool calls lowered as assistant(toolUse x2), user(toolResult 1), user(toolResult 2). A live request against Amazon Nova Micro returned HTTP 400: Expected toolResult blocks at messages.2.content for the following Ids: weather_london.

After: The same canonical history lowers as assistant(toolUse x2), user(toolResult 1, toolResult 2). The recorded live request returned HTTP 200 and the model continued with the requested answer, Paris is sunny; London is rainy.

How

• packages/ai/src/protocols/bedrock-converse.ts appends lowered canonical user and tool content to an immediately preceding Bedrock user message.
• packages/ai/test/provider/bedrock-converse.test.ts covers the lowered wire shape and replays a real Bedrock continuation after two parallel tool results.
• packages/ai/test/fixtures/recordings/bedrock-converse/continues-after-parallel-tool-results.json contains the redacted HTTP 200 Bedrock exchange.

Scope

This only changes Bedrock Converse message lowering. It does not change canonical message construction or lowering for other providers.

Testing

• Live before-fix reproduction at parent commit 964f7f4254: Bedrock HTTP 400 identifying the missing second tool result in messages.2.content.
• Live after-fix recording with RECORD=true RECORDED_TEST="continues after parallel tool results" bun run test test/provider/bedrock-converse.test.ts: Bedrock HTTP 200 and exact final answer.
• Replay with bun run test test/provider/bedrock-converse.test.ts: 44 pass, 0 fail.
• bun typecheck from packages/ai: passes.
• Prettier and git diff --check: pass.
• Push hook bun turbo typecheck --concurrency=3: 33 tasks pass.