fix: handle Mistral Magistral structured content blocks

[teknium1]

Summary

Mistral Magistral reasoning models (magistral-medium-latest, magistral-small-latest) and mistral-large-2512+ return message.content as a list of structured blocks instead of a plain string:

"content": [
  {"type": "thinking", "thinking": [{"type": "text", "text": "reasoning..."}]},
  {"type": "text", "text": "final answer"}
]

This happens in both streaming deltas and non-streaming responses, causing TypeError: sequence item 0: expected str instance, list found when the code tries to join content_parts.

Reported by Joseph C. on Discord.

Changes

• Add _normalize_structured_content() helper that extracts text and thinking parts from Mistral structured blocks
• Streaming path: normalize delta.content before appending to content_parts, route thinking blocks to reasoning_parts with proper callback firing
• Non-streaming normalization: replace the simple list handler with the full helper that also extracts thinking blocks as reasoning_content (was silently dropping them)
• _build_assistant_message: normalize list content early before string operations and regex matching
• Length truncation/continuation: normalize content before string concatenation

Test plan

• 25 new tests in tests/test_mistral_structured_content.py covering:
  • Helper function (13 tests): all block types, edge cases, string passthrough
  • _build_assistant_message (6 tests): list content, tool calls, deduplication
  • Non-streaming normalization (2 tests): list and dict content
  • Streaming simulation (4 tests): join safety, mixed chunks, thinking-only deltas
• E2E test with real imports from worktree and isolated HERMES_HOME
• Full suite: 5001 passed (4 pre-existing failures unrelated)

[teknium1]

Closing during PR triage — not pursuing this approach.