⚡ feat: Add Gemini 3.8 Flash Support (#15519)

* ⚡ feat: Add Gemini 3.8 Flash Support

Adds first-class support for Google's Gemini 3.8 Flash (`gemini-3.8-flash`,
GA 2026-09-02) for both the Gemini API (AI Studio) and Google Cloud Gemini
Enterprise Agent Platform, following the Gemini 3.7 Flash integration (#14818).

The Flash-family handler that PR generalized already carries the shape 3.8
needs, so registering the model is a one-line rule rather than new behavior:
it inherits the strip of deprecated sampling params (temperature/topP/topK),
the rejected penalty params, and `thinkingBudget`, defaults to `medium`
thinking, and — like 3.7 — errors on `minimal`, so an explicit `minimal` is
substituted with the nearest supported level, `low`.

- Context window (1,048,576) in googleModels; API + cache pricing in tx.ts
- Model dropdown (config.ts) and GOOGLE_MODELS examples for both integrations
- Register the model in the Flash-family thinking-rule table
- Apply the same introductory pricing as 3.6/3.7 Flash ($0.75 in / $3.75 out /
  $0.075 cached, per 1M), reverting to $1.50 / $7.50 / $0.15 on 2027-01-01;
  the existing comments at both call sites now name 3.8 alongside 3.6/3.7
- Tests mirroring the 3.7 Flash coverage: thinking default, legacy-param strip,
  explicit level pass-through, `minimal` substitution, versioned aliases,
  context window, model-key mapping, and rates

Refs #15516

Ref: https://ai.google.dev/gemini-api/docs/models/gemini-3.8-flash
Ref: https://ai.google.dev/gemini-api/docs/pricing

* 🔧 chore: Update @librechat/agents dependency to version 3.7.15 in package-lock.json and related package.json files