feat(claude-agent-sdk): opt-in multi-block tool-result concatenation

[boyuan-fh]

Fixes #2632

Summary

buildAguiToolMessage reads only content[0], silently dropping everything
after the first block for tools returning multiple content blocks. Rather than
changing the default behavior, this adds a concatenateToolResultBlocks config
option (default false) so callers can opt in.

• concatenateToolResultBlocks: false (default): exactly the current
  content[0]-only behavior. No behavioral change for existing consumers.
• concatenateToolResultBlocks: true: iterates all content blocks. Text
  blocks are joined with \n; mixed content (text + non-text blocks) falls
  back to JSON.stringify(content) to avoid silently dropping non-text blocks
  like images.

The JSON round-trip (JSON.stringify(JSON.parse(text))) is preserved in both
paths for parity with the Python sibling adapter. Whether to remove it is an
open question (happy to drop the round-trip in favor of verbatim pass-through if you prefer).

Config option

• Name: concatenateToolResultBlocks
• Type: boolean
• Default: false
• Location: ClaudeAgentAdapterConfig in src/types.ts
• Threading: adapter.ts reads this.config.concatenateToolResultBlocks
  and passes it as the third argument to buildAguiToolMessage(toolUseId, resultContent, concatenate).

Test plan

• New src/utils.buildAguiToolMessage.test.ts with 16 cases in two groups:

  Shared edge cases (independent of flag):

  • Empty content array serialized as "[]"
  • Null content returns empty string
  • Non-array content serialized via JSON.stringify
  • Correct id format (${toolUseId}-result)

  Default (concatenate disabled):

  • Multi-block input yields only block 0
  • Single text block passes through unchanged
  • Non-JSON text passes through without modification
  • Non-text first block falls back to JSON.stringify(content)
  • JSON text is re-serialized (round-trip behavior)

  Opt-in (concatenate enabled):

  • Multi-block text result concatenates all blocks with \n
  • Single text block passes through unchanged
  • Non-JSON text passes through without modification
  • Non-text blocks fall back to JSON.stringify(content)
  • Mixed text + non-text blocks: lossless JSON.stringify(content)
  • Non-text-first mixed content ([image, text]): image survives
  • JSON text is re-serialized (round-trip behavior)