Add Copilot AI-credits fallback pricing and skip AI-credits guardrails for BYOK

[Copilot]

Copilot runs routed through AWF were hard-failing before the first turn when the resolved model had no AI-credits pricing entry (model: auto and other unknown models), and BYOK Copilot requests were incorrectly hitting the same pricing gate even though they do not consume GitHub AI credits.

• Config generation: emit fallback pricing for unresolved Copilot models

  • Extend generated apiProxy config to include defaultAiCreditsPricing.
  • Use a conservative default for Copilot-backed requests so unresolved aliases/models no longer 400 at the proxy.

• BYOK handling: disable AI-credits enforcement for external providers

  • Detect Copilot BYOK mode from COPILOT_PROVIDER_* engine env.
  • Omit apiProxy.maxAiCredits for BYOK runs.
  • Skip runtime GH_AW_MAX_AI_CREDITS injection and token-steering when BYOK is active, so requests to user-owned endpoints do not enter the AI-credits pricing path.

• Coverage updates

  • Add focused tests for:
    • fallback pricing in generated Copilot AWF config
    • BYOK omission of AI-credits guardrail config
    • BYOK omission of runtime max-AIC injection
  • Refresh Copilot WASM goldens to match the new generated config output.

Example of the generated apiProxy shape after this change for standard Copilot runs:

{
  "apiProxy": {
    "enabled": true,
    "maxAiCredits": "${GH_AW_MAX_AI_CREDITS}",
    "defaultAiCreditsPricing": {
      "input": 3.0,
      "output": 15.0,
      "cachedInput": 0.3,
      "cacheWrite": 3.75
    }
  }
}

For Copilot BYOK runs, maxAiCredits and defaultAiCreditsPricing are intentionally omitted so the request bypasses GitHub AI-credits enforcement entirely.

• Fixes [aw-failures] [aw-fix] P1: Copilot auto model has no AI-credits pricing — api-proxy returns 400 and fails Smoke Copilot / AI-c [Content truncated due to length] #46846