fix(api): stop JSON.stringify-ing messages before estimateTokens (restores #8368 image estimate)

[Prudhvivuda]

Summary

Six production call sites pre-JSON.stringify() the messages/body before passing them to estimateTokens(), forcing the char/4 text path and undoing the #8368 inline-base64-image bounded estimate for those paths. estimateTokens(string) takes Math.ceil(str.length / CHARS_PER_TOKEN); the image-detection walk (extractImageTokens) only runs for the object overload introduced by #8368. Stringifying first strips the structural type information the fix depends on.

Impact — for any Chat Completions request carrying inline base64 images ({ type: 'image_url', image_url: { url: 'data:image/...' } }) that triggers compression:

1. compressContext over-estimates (a ~500 KB image → ≈125 000 tokens via base64-as-text instead of the correct ~1 200-token estimate) → thinks the context is way over the limit and enters aggressive compression.
2. purifyHistory binary-search prunes image-bearing turns unnecessarily → silent context loss.
3. combo.ts fallback compression fires for requests that fit comfortably.

Fix

Pass the structured object directly at all six sites — estimateTokens already handles both the string and object overloads; the object path walks the structure for inline base64 image blocks, substitutes the bounded per-image estimate, then measures the remainder as text.

Sites fixed:

• open-sse/services/contextManager.ts — compressContext initial estimate + after Layers 1/2/3, and the purifyHistory binary-search candidate check (5 sites).
• open-sse/services/combo.ts — fallback compression threshold check (1 site).

Test plan

New TDD guard tests/unit/8594-compress-image-token-stringify.test.ts drives compressContext directly (fails before the fix, passes after):

• a within-limit inline-image request must not be compressed;
• purifyHistory retains all image-bearing turns when they fit;
• an oversized text request is still compressed (control — no regression).

Validated on Node v22.8.0 (supported range):

• New suite: 3/3 pass (2 of 3 fail on base 4053e2314, confirming reproduction).
• #8368 image-token suite: 8/8 pass (no regression).
• Compression/context suites (context-manager, compression-pipeline-inflation-guard, compression-noop-guard, combo-context-length, context-window-reconcile, compression-tokens): 54/54 pass.
• npm run typecheck:core clean; eslint clean on all changed files.

Closes #8594
Refs #8368, #8560

[mergify]

⚠️ The sha of the head commit of this PR conflicts with #7076. Mergify cannot evaluate rules on this PR. Once #7076 is merged or closed, Mergify will resume processing this PR. ⚠️