Show file collision preview in init instead of generic non-empty warning

[Copilot]

PromptIfNonEmpty showed a blanket "WARNING: The current directory is not empty" before the template was even downloaded, so it couldn't know which files would actually collide. This fired even when no files would be overwritten, and never listed the specific collisions.

Changes

• PromptIfNonEmpty → no-op: Removed the generic warning and confirmation prompt. promptForDuplicates (called after template staging) already handles collision detection with full file information.
• promptForDuplicates warning update: Warning now reads "The current directory is not empty. Initializing an app in this directory may overwrite existing files:" and lists colliding file paths in red (output.WithErrorFormat). No warning is shown when there are zero collisions.
• Tests updated: TestInitializer_PromptIfNonEmpty simplified to verify no-op. New TestPromptForDuplicates_ShowsCollisions covers both the no-collision (silent) and collision (warning + file list) cases.

Before/After

When no files collide: no warning shown (previously showed generic warning).

When files collide:

WARNING: The current directory is not empty. Initializing an app in this directory may overwrite existing files:
  README.md          ← red
  src/agent.yaml     ← red
? What would you like to do with these files?

[microsoft-github-policy-service]

Hi @@copilot. Thank you for your interest in helping to improve the Azure Developer CLI experience and for your contribution. We've noticed that there hasn't been recent engagement on this pull request. If this is still an active work stream, please let us know by pushing some changes or leaving a comment. Otherwise, we'll close this out in 7 days.

[jongio]

The approach makes sense - promptForDuplicates already detects exact collisions after the template is staged, so the early blanket warning in PromptIfNonEmpty was firing even when no files would actually collide. Removing it and improving the collision message is the right call.

A few things to address:

1. AZD_ALLOW_NON_EMPTY_FOLDER is now dead: the env var check lived in PromptIfNonEmpty (now a no-op). The constant at line 820, the stripping in InitEnvFileValues(), and the docs in environment-variables.md should be cleaned up. Users who set it in CI won't break (the prompt is gone for everyone), but dead plumbing is confusing.

2. Merge conflicts: the PR can't merge as-is. Needs a rebase onto current main.

3. Nit: existing tests in this file are gradually moving to t.Context() (8 uses already) per Go 1.26 conventions - the new test code could follow suit instead of context.Background().

[jongio]

AZD_ALLOW_NON_EMPTY_FOLDER (line 820) was the only consumer of the check that lived here. Now that this is a no-op, that env var has no effect anywhere. Worth cleaning up the constant, the InitEnvFileValues() stripping logic, and docs/environment-variables.md in the same PR.