fix: handle current Perplexity SSE responses

[ardzz]

Description

Fix MCP calls that completed upstream but returned an empty answer to OpenCode.

Root cause

Perplexity's current SSE response shape emits the answer in a plain ask_text.markdown_block and may finish without the legacy outer FINAL event. The client only handled legacy FINAL and limited diff-block fallbacks, so valid answers could be discarded as No response received..

What changed

• recognize current ask_text, ask_text_markdown, and numbered markdown answer blocks
• treat direct markdown_block.answer as authoritative while preserving legacy diff and FINAL parsing
• collect and URL-deduplicate current web_results citations, including incremental patches
• stop on valid terminal signals and always close the stream generator and HTTP response
• add bounded MCP timeouts with explicit partial/error responses:
  • search: thinking model, mode="search", 45 seconds
  • research: thinking model, mode="copilot", 180 seconds
  • accepted override range: 5–600 seconds
• remove the ambiguous No response received. fallback
• add regression coverage for current/legacy schemas, conflicting snapshots, numbered sections, citations, timeout behavior, resource cleanup, and FastMCP schema defaults

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Other (please describe):

Testing

• I have tested these changes locally
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Validation performed:

• 572 passed in the non-live test suite
• Pyright: 0 errors and 0 warnings
• Ruff fatal/import checks: passed
• git diff --check: passed
• OpenCode-style MCP stdio smoke (initialize → tools/list → tools/call): succeeded with a non-empty response in 8.36 seconds
• independent code review: approved with no remaining Critical, Important, or Minor findings

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

N/A

Additional Notes

No documentation or dependent-package changes are required. The feature worktree is intentionally preserved for PR feedback. The unrelated local uv.lock modification on master was not staged, committed, or pushed.

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