… auto-precision display (#423)

* fix: cache_tokens extraction in non-codex upstreams + image_generation request counters

Three orthogonal dashboard accuracy issues, fixed together:

1) **Cache hit rate stuck at 0%** — OpenAI / Anthropic / Gemini upstream
   adapters were synthesizing `response.completed` with hardcoded
   `input_tokens_details: {}`, dropping the cache hit info that the native
   APIs do return. Now extract `prompt_tokens_details.cached_tokens`
   (OpenAI), `cache_read_input_tokens` (Anthropic message_start +
   message_delta), and `cachedContentTokenCount` (Gemini explicit caches),
   and surface them under the standard Codex shape so the existing parsers
   pick them up.

2) **No image generation request counter** — PR #422 added image token
   tracking but no count of image requests (success vs failure). Now
   detect `tools[].type === "image_generation"` at request parse time,
   propagate `expectsImageGen` through proxy-handler, and on every
   release call site (success / EmptyResponse / upstream errors)
   classify as success (image_output_tokens > 0) or failed. Adds:
   - AccountUsage: image_request_count, image_request_failed_count
     (+ window mirrors)
   - UsageSnapshot/Baseline/DataPoint/Summary: same
   - /admin/usage-stats/summary: total_image_request_count,
     total_image_request_failed_count
   - Dashboard "Image Requests" card showing N ok · M failed
   - AccountCard window image requests row when activity present

3) **Hit rate display rounded sub-0.05% values to 0.0%** — formatHitRate
   now uses auto precision: ≥1% one decimal, 0.01-1% two decimals,
   >0 but <0.01% shows "<0.01%", =0 shows "0%".

Backward compat: old usage-history.json snapshots without the new
image_request fields read as 0 via `?? 0`. Tests:
- 7 new unit tests for upstream cache extraction (OpenAI/Anthropic/Gemini)
- 5 new unit tests for image_request counter logic in recordUsage
- 2 new unit tests for usage-stats image_request aggregation
- tests/real/image-generation.test.ts e2e block now asserts
  total_image_request_count == +1 after a single successful gen

* fix: harden cache-token extraction + count compact-route image_gen attempts

Self-review followups on PR #423:

- Anthropic message_delta: take Math.max(start, delta) for
  cache_read_input_tokens so a future API change emitting 0 in delta
  can't clobber a real hit reported in message_start. Adds defensive
  unit test covering the regression.
- /v1/responses/compact (handleCompact): detect image_generation tool
  at request time and synthesize a `image_request_attempted=true,
  image_request_succeeded=false` usage on every release site. Compact
  doesn't surface tool_usage.image_gen, so any image_generation tool
  forwarded here is always classified as failed — at least the
  dashboard now catches accidental misuse rather than silently dropping
  the signal.

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Co-authored-by: icebear0828 <icebear0828@users.noreply.github.com>