fix(api): replace phrase-based reasoning sanitizer with tag-based filter (#779)

Reasoning models (MiniMax M2.7, GLM-4.5/5, DeepSeek, Kimi K2) inline
chain-of-thought inside <think>...</think> tags in the content field
rather than using the reasoning_content channel. The prior phrase-matching
sanitizer (looksLikeLeakedReasoningPrefix) only caught English-prose
preambles like "I should"/"the user asked", missed tag-based leaks
entirely, and risked false-stripping legitimate assistant output.

Replace with a structural tag-based approach (same pattern as hermes-agent):

- createThinkTagFilter() — streaming state machine that buffers partial
  tags across SSE delta boundaries (<th| + |ink>), so tags split mid-chunk
  still parse correctly.
- stripThinkTags() — whole-text cleanup for non-streaming responses and
  as a safety net. Handles closed pairs, unterminated opens at block
  boundaries, and orphan tags.
- Recognizes think, thinking, reasoning, thought, REASONING_SCRATCHPAD
  case-insensitively, including tags with attributes.
- False-negative bias: flush() discards buffered partial tags at stream
  end rather than leaking them.

Existing phrase-based shim tests updated to exercise the actual <think>
tag leak. Added regression tests confirming legitimate prose starting
with "I should..." is preserved (the old sanitizer's main false-positive).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/services/api/codexShim.ts

@@ -6,10 +6,9 @@ import type {
 } from './providerConfig.js'
 import { sanitizeSchemaForOpenAICompat } from './openaiSchemaSanitizer.js'
 import {
-  looksLikeLeakedReasoningPrefix,
-  shouldBufferPotentialReasoningPrefix,
-  stripLeakedReasoningPreamble,
-} from './reasoningLeakSanitizer.js'
+  createThinkTagFilter,
+  stripThinkTags,
+} from './thinkTagSanitizer.js'
 
 export interface AnthropicUsage {
   input_tokens: number
@@ -734,25 +733,22 @@ export async function* codexStreamToAnthropic(
     { index: number; toolUseId: string }
   >()
   let activeTextBlockIndex: number | null = null
-  let activeTextBuffer = ''
-  let textBufferMode: 'none' | 'pending' | 'strip' = 'none'
+  const thinkFilter = createThinkTagFilter()
   let nextContentBlockIndex = 0
   let sawToolUse = false
   let finalResponse: Record<string, any> | undefined
 
   const closeActiveTextBlock = async function* () {
     if (activeTextBlockIndex === null) return
-    if (textBufferMode !== 'none') {
-      const sanitized = stripLeakedReasoningPreamble(activeTextBuffer)
-      if (sanitized) {
-        yield {
-          type: 'content_block_delta',
-          index: activeTextBlockIndex,
-          delta: {
-            type: 'text_delta',
-            text: sanitized,
-          },
-        }
+    const tail = thinkFilter.flush()
+    if (tail) {
+      yield {
+        type: 'content_block_delta',
+        index: activeTextBlockIndex,
+        delta: {
+          type: 'text_delta',
+          text: tail,
+        },
       }
     }
     yield {
@@ -760,8 +756,6 @@ export async function* codexStreamToAnthropic(
       index: activeTextBlockIndex,
     }
     activeTextBlockIndex = null
-    activeTextBuffer = ''
-    textBufferMode = 'none'
   }
 
   const startTextBlockIfNeeded = async function* () {
@@ -837,43 +831,17 @@ export async function* codexStreamToAnthropic(
 
     if (event.event === 'response.output_text.delta') {
       yield* startTextBlockIfNeeded()
-      activeTextBuffer += payload.delta ?? ''
       if (activeTextBlockIndex !== null) {
-        if (
-          textBufferMode === 'strip' ||
-          looksLikeLeakedReasoningPrefix(activeTextBuffer)
-        ) {
-          textBufferMode = 'strip'
-          continue
-        }
-
-        if (textBufferMode === 'pending') {
-          if (shouldBufferPotentialReasoningPrefix(activeTextBuffer)) {
-            continue
-          }
+        const visible = thinkFilter.feed(payload.delta ?? '')
+        if (visible) {
           yield {
             type: 'content_block_delta',
             index: activeTextBlockIndex,
             delta: {
               type: 'text_delta',
-              text: activeTextBuffer,
+              text: visible,
             },
           }
-          textBufferMode = 'none'
-          continue
-        }
-
-        if (shouldBufferPotentialReasoningPrefix(activeTextBuffer)) {
-          textBufferMode = 'pending'
-          continue
-        }
-        yield {
-          type: 'content_block_delta',
-          index: activeTextBlockIndex,
-          delta: {
-            type: 'text_delta',
-            text: payload.delta ?? '',
-          },
         }
       }
       continue
@@ -969,7 +937,7 @@ export function convertCodexResponseToAnthropicMessage(
         if (part?.type === 'output_text') {
           content.push({
             type: 'text',
-            text: stripLeakedReasoningPreamble(part.text ?? ''),
+            text: stripThinkTags(part.text ?? ''),
           })
         }
       }