fix: strip Anthropic-specific params from 3P provider paths

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.

src/services/api/openaiShim.ts

@@ -195,10 +195,12 @@ function convertContentBlocks(
         // handled separately
         break
       case 'thinking':
-        // Append thinking as text with a marker for models that support reasoning
-        if (block.thinking) {
-          parts.push({ type: 'text', text: `<thinking>${block.thinking}</thinking>` })
-        }
+      case 'redacted_thinking':
+        // Strip thinking blocks for OpenAI-compatible providers.
+        // These are Anthropic-specific content types that 3P providers
+        // don't understand. Serializing them as <thinking> text corrupts
+        // multi-turn context: the model sees the tags as part of its
+        // previous reply and may mimic or misattribute them.
         break
       default:
         if (block.text) {