Add Copilot pre-flight diagnostic for GHES environments (#20975)

* Initial plan

* Completing task

Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

* Add Copilot pre-flight diagnostic for GHES environments

Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

* fix: update tests for copilot pre-flight diagnostic step

Update all Copilot engine tests to account for the new pre-flight
diagnostic step (2 steps instead of 1). Extract a shared helper
for finding the Copilot execution step, and regenerate WASM golden
files to include the new step.

Fixes:
- TestFirewallArgsInCopilotEngine
- TestFirewallBlockedDomainsInCopilotEngine
- TestFirewallLogLevelInCopilotEngine
- TestChrootModeInAWFContainer
- TestChrootModeEnvFlags
- TestMCPScriptsWithFirewallIncludesHostDockerInternal
- TestEngineAWFEnableApiProxy
- TestWasmGolden_CompileFixtures (golden files)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fix incorrect step count assertions in preflight diagnostic tests (#20985)

* Initial plan

* fix: correct test conditions for preflight step count checks

Fix two categories of incorrectly written tests introduced with the
Copilot pre-flight diagnostic step:

1. copilot_engine_test.go: Two "skips preflight" sub-tests checked
   `len(steps) != 2` but preflight is intentionally skipped, returning
   only 1 step. Change condition to `!= 1`.

2. engine_agent_import_test.go: Claude and Codex tests expected 2 steps
   (preflight + execution) like Copilot, but those engines only return 1
   execution step. Change to `!= 1` and use `steps[0]`.

Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

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Co-authored-by: anthropic-code-agent[bot] <242468646+Claude@users.noreply.github.com>
Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>
Co-authored-by: Landon Cox <landon.cox@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>

Author: 4 people