`azd ext install`: allow installing directly from a registry URL/location without a separate `ext source add` step

[JeffreyCA]

Summary

Today, installing an extension from a custom registry is a two-step flow:

azd extension source add -n my-source -t url -l https://link/to/registry.json
azd extension install <id> -s my-source

We'd like a one-liner that accepts a registry location directly:

azd extension install <id> -s https://link/to/registry.json

When -s/--source is given a URL or file path (rather than the name of an already-registered source), azd registers the source and then installs the extension from it.

Motivation

The Azure SDK bot posts an "Azure Dev CLI Install Instructions" comment on azure-dev PRs with installer links so reviewers/end-users can quickly install and test a PR build of azd without building from source.

We want the equivalent for extension PRs (tracked separately, see related issue): a PR pipeline publishes the extension build to a temporary/custom registry, and the bot comments a single copy-paste command. The current two-step source add + install flow is more friction than necessary, and the intermediate source name is an implementation detail users shouldn't have to invent.

Also complements /pull/8740, to easily install nightlies.

Also complements /pull/8697, which supports installing extension zips/bundles.

Proposed behavior

Detect whether the -s/--source value is a location (URL or file path) vs. the name of an existing source:

• Existing source name → current behavior, unchanged.
• Location → register the source (inferring type as url vs file the same way source add does), then install <id> from it.

Details:

• Source is persisted, not ephemeral — so azd extension upgrade/list/show continue to work against that registry afterward without re-specifying the URL.
• Prompt for the source name. Under --no-prompt, direct user to add source separately beforehand.
• Confirm before registering when installing directly from a URL, since it may be an untrusted source.

Acceptance criteria

• azd extension install <id> -s <url-or-path> installs the extension without a prior azd extension source add.
• The source is persisted with the chosen name and inferred type.
• User is prompted for the source name and confirms before a URL source is registered.
• Existing behavior for -s <existing-source-name> is unchanged.

Related

• Adding in support for making any extension pipeline a "nightly" publishing pipeline #8740
• Self-contained extension bundles (azd x pack --bundle and azd ext install <bundle.zip>) #8697