[typespec-python-regenerate workflow] Harden TypeSpec Python README warning workflow

[msyyc]

Summary:

• Remove the generated README template file dependency from the TypeSpec Python regeneration workflow.
• Generate the short test-package README warning inline before copying it to generated package READMEs.
• Remove stale template-specific guidance from the generated-tests branch error message.

Validation:

• git diff --check -- .github/workflows/typespec-python-regenerate.yml
• VS Code YAML diagnostics: no errors
• Local Git Bash temp-tree check for the simplified README warning copy flow

Test: https://github.com/Azure/azure-sdk-for-python/actions/runs/28645359471

[Copilot]

Pull request overview

This PR hardens the TypeSpec Python Regenerate Tests workflow by eliminating its dependency on a checked-in README template file. Previously the workflow copied eng/tools/azure-sdk-tools/emitter/generated/template/README.md onto every generated test-package README (and had to guard the template path during the find). Now the short "test code — do not install from PyPI" warning is generated inline into a temp file, removing the external file dependency and simplifying the copy logic.

Changes:

• Replaced the template-file copy with an inline heredoc that writes the warning to a mktemp temp file (cleaned up via an EXIT trap).
• Dropped the now-unnecessary ! -path "$TARGET/template/*" exclusion from the find, keeping only the top-level generated/README.md exclusion.
• Removed stale template-specific guidance from the missing-branch error message.