feat(cli): support shadowing to an existing deployment (DX-831)

[blainekasten]

Summary

• tg beta endpoints shadow can attach an existing deployment as a shadow target by passing a dep_... (or deployment name) as the first positional ENDPOINT arg — parent endpoint is inferred.
• No new flag: dropped --target-deployment-id per review feedback.
• Create-new-deployment path unchanged: ENDPOINT + MODEL.

Usage

# Attach an existing deployment
tg beta endpoints shadow dep_xxxxxxxxxxxx --rate 0.1

# Create a new shadow deployment (unchanged)
tg beta endpoints shadow my-endpoint Qwen/Qwen2.5-7B --rate 0.1

Test plan

• uv run pytest tests/cli/test_beta_endpoints_shadow.py -v (34 passed)
• Unit coverage for dep_ vs ep_ / MODEL create-path detection
• CLI coverage for attach existing deployment, custom target name, experiment reuse, not-found, MODEL+dep_ rejection
• Existing create-new-deployment shadow path still green
• Help epilogue shows tg beta endpoints shadow dep_... --rate 0.1

Fixes DX-831

Linear Issue: DX-831

  

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[blainekasten]

@cursoragent I don't want to add a new flag for this. Drop --target-deployment-id and let it come in on the first positional arg

[cursor]

Dropped --target-deployment-id. Existing deployments go on the first positional:

tg beta endpoints shadow dep_xxxxxxxxxxxx --rate 0.1

Endpoint is inferred from the deployment. Create path unchanged (endpoint + MODEL). 34 tests green; PR updated.