Three silent failure modes affecting all third-party provider users:
1. Thinking blocks serialized as <thinking> text corrupt multi-turn
context — strip them instead of converting to raw text tags.
2. Unknown models fall through to 200k context window default, so
auto-compact never triggers — use conservative 8k for unknown
3P models with a warning log.
3. Session resume with thinking blocks causes 400 or context corruption
on 3P providers — strip thinking/redacted_thinking content blocks
from deserialized messages when resuming against a non-Anthropic
provider.
Addresses findings 2, 3, and 5 from #248.
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