eng, Allow preserving designated libraries during SDK sync

[Weidong Xu (weidongxu-microsoft)]

Summary

• Add an UpgradeDesignatedLibraries pipeline boolean, defaulting to true.
• Pass it to sync_emitter.py as --upgrade-designated-libraries.
• Cache designated library versions before emitter configuration is regenerated.
• Restore the cached versions when upgrades are disabled.

By default, behavior is unchanged: libraries sourced from azure-rest-api-specs or npm latest are upgraded. Disabling the option isolates an emitter regeneration from independently versioned designated library updates.

Validation

• Verified disabled mode restores all cached designated versions without querying specs or npm for them.
• Verified enabled mode resolves designated versions from their existing configured sources.
• Parsed the pipeline YAML and compiled the Python script.

For #50481, where sync openai-typespec lib from azure-rest-api-specs caused large diff

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[Copilot]

🟢 Approval recommended

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Pull request overview

This PR adds an opt-out for upgrading OpenAI TypeSpec during emitter synchronization while preserving the existing default behavior.

Changes:

• Adds and forwards the UpgradeOpenAITypeSpec pipeline option.
• Preserves the existing OpenAI TypeSpec pin when disabled.

File summaries

File	Description
eng/pipelines/scripts/sync_emitter.py	Implements optional OpenAI TypeSpec upgrade behavior.
eng/pipelines/post-publish-emitter.yaml	Adds and passes the pipeline parameter.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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