Run optional post-emitter.ps1 script after SDK generation

[msyyc]

for microsoft/typespec#10230

Summary

Adds support for an optional, hard-coded post-emitter PowerShell script that runs after SDK code generation. When a script named _post_emitter.ps1 exists directly inside a generated package folder (sdk/<service>/azure-*), the SDK generation pipeline (sdk_generator.py) executes it. This lets service teams (e.g. Foundry) run custom post-processing on the generated SDK.

The leading underscore in _post_emitter.ps1 signals that the script is private to the package.

This is intentionally not language-specific in spirit; each language repo owns its own pipeline, so the same one-line hook can be replicated to other language repos once validated for Python.

Behavior

• Looks for _post_emitter.ps1 in the package folder after code generation / metadata update and before ApiView + build, so downstream steps pick up any changes.
• Runs only if the script is present; otherwise it is a no-op.

Safety / robustness

• Non-interactive invocation: pwsh -NonInteractive -NoProfile -ExecutionPolicy Bypass -File.
• Output logged: stdout/stderr are captured and written to the pipeline log (stdout as INFO, stderr as WARNING).
• Timeout: 600s, so a hung script cannot stall the pipeline.
• Failure isolation: all failures (non-zero exit, timeout, exceptions) are logged as warnings and never fail the overall generation.
• Graceful degradation: if no PowerShell (pwsh/powershell) executable is available, it logs a warning and skips.

Note: PowerShell has no built-in way to confine a script's file writes to a folder. If stronger isolation is needed, it must come from the OS layer (filesystem ACLs / dedicated low-privilege user, or a container/sandbox mounting only the package folder read-write). A git-diff post-check could also be added to detect writes outside the package folder.

Testing

Validated locally with a temp package folder containing a _post_emitter.ps1: script executes, stdout/stderr are logged, and the no-script case is a no-op.

[Copilot]

Pull request overview

This PR extends the eng/tools/azure-sdk-tools SDK generation pipeline by adding a post-generation hook: if a post-emitter.ps1 script exists directly in a generated package folder, sdk_generator.py will execute it after codegen/metadata updates and before ApiView/build steps, enabling service-specific post-processing.

Changes:

• Added run_post_emitter_script() to detect and execute an optional post-emitter.ps1 in the generated package directory with logging, timeout, and failure isolation.
• Wired the new hook into main() immediately after sdk_update_metadata(...), ensuring downstream steps see any script-produced changes.

[iscai-msft]

@copilot change the POST_EMITTER_SCRIPT_NAME variable value from _post_emitter.ps1 to PostEmitter.ps1