Prefer compat toolcache fallback for explicit Copilot CLI versions

[Copilot]

Explicit Copilot CLI pins were forcing exact-version toolcache lookup, which bypassed compat-range matching and turned a compatible cached runner binary into an unconditional network download. This kept agentic jobs exposed to github.com/CDN availability even when the runner already had an in-window Copilot CLI installed.

• What changed

  • Explicit engine.version installs now still receive GH_AW_COMPILED_VERSION, so the installer can resolve the compat window at runtime.
  • install_copilot_cli.sh now treats an explicit version as:
    1. exact toolcache match if present,
    2. otherwise compat-window toolcache fallback,
    3. otherwise release download.
  • The no-explicit-version path remains unchanged: compat resolution still selects the window and downloads only when no cached candidate satisfies it.

• Installer behavior

  • Compat resolution is now attempted for explicit versions when the compiled gh-aw version is available.
  • Exact pins still win when the matching binary exists in toolcache.
  • If the exact pin is absent but the runner has a compatible cached binary within min-agent..max-agent, that cached binary is used instead of downloading from GitHub.

• Coverage added

  • Step generation test for explicit version + compiled version env injection.
  • Installer script test proving explicit version requests can fall back to a compat-window cached binary.

• Example

  # Before:
  # explicit 1.0.75 + cached 1.0.56 => reject cache, download 1.0.75
  
  # After:
  # explicit 1.0.75 + compat window 1.0.21..1.0.75 + cached 1.0.56
  # => no exact hit, accept cached 1.0.56 via compat fallback, skip download

• Net effect

  • Restores the toolcache fast path for explicit Copilot version requests.
  • Reduces hard dependency on release downloads in agent/detection jobs when the runner already has a compatible CLI cached.

• Fixes DefaultCopilotVersion drifts past compat.json max-agent, forcing a network install on every job and disabling the toolcache path #48358