💎 fix: Stop Double-Counting Cache Tokens for Gemini/OpenAI in Usage Spend (#12868)

* 💎 fix: Stop Double-Counting Cache Tokens for Gemini/OpenAI in Usage Spend (#12855)

Different providers report `usage_metadata.input_tokens` with different
semantics:

  - Anthropic / Bedrock: `input_tokens` EXCLUDES cache; cache reads/writes
    arrive separately and must be added to get the total prompt size.
  - Gemini / OpenAI: `input_tokens` ALREADY INCLUDES cached tokens
    (Google's `promptTokenCount`, OpenAI's `prompt_tokens`). Their
    `input_token_details.cache_*` are subsets of `input_tokens`.

`recordCollectedUsage` treated both schemes as additive, so for cache-hit
requests on Gemini/OpenAI it added cache tokens on top of an
`input_tokens` value that already contained them — overcharging users by
the cache_hit_rate (e.g., ~67% cache hit ≈ 1.67x overcharge). This
matches the issue reporter's GCP billing comparison.

Adds a small `splitUsage` helper that classifies the provider by model
name and computes `inputOnly` (the non-cached portion) plus the
all-inclusive `totalInput` for both the spend math and the returned
`input_tokens` summary. The helper defaults to additive semantics (the
historical behavior) so unknown providers are unaffected.

Updates existing OpenAI-shaped tests that previously asserted the buggy
additive math, and adds Gemini regression tests using the exact numbers
from the issue report (input=11125, cache_read=7441 → input=3684).

Anthropic / Bedrock paths remain bit-identical to before.

* 🔧 refactor: Classify Cache-Token Semantics by Provider, Not Model Name

Follows up the previous commit. Replaces a model-name regex
(`gemini|gpt|o[1-9]|chatgpt`) with an explicit `Providers` enum lookup
keyed off the `usage.provider` field — `UsageMetadata.provider` already
exists in `IJobStore.ts` but was never being populated.

  - `callbacks.js#ModelEndHandler` now attaches `usage.provider` from
    `agentContext.provider` alongside `usage.model`.
  - `usage.ts` uses a `SUBSET_PROVIDERS` set (`openAI`, `azureOpenAI`,
    `google`, `vertexai`, `xai`, `deepseek`, `openrouter`, `moonshot`)
    backed by the canonical `Providers` enum from
    `librechat-data-provider`.
  - `xai`, `deepseek`, `openrouter`, `moonshot` extend `ChatOpenAI` so
    they inherit subset semantics (verified in node_modules).
  - Defaults to additive when `usage.provider` is missing, so the title
    flow (which doesn't propagate provider) and any pre-this-PR usage
    entries keep their existing behavior.

Tests: switch fixtures from model-name signaling to explicit `provider`
field, plus a Vertex AI case and a "missing provider" fallback case.

api/server/controllers/agents/callbacks.js

@@ -75,6 +75,9 @@ class ModelEndHandler {
       if (modelName) {
         usage.model = modelName;
       }
+      if (agentContext.provider) {
+        usage.provider = agentContext.provider;
+      }
 
       const taggedUsage = markSummarizationUsage(usage, metadata);