azd x: non-fatal metadata warnings and azd x init wizard polish

[Copilot]

This PR makes azd x build extension metadata warnings non-fatal so they no longer abort azd x init, and rolls in several azd x init wizard fixes including Issue #8207.

Warning handling

azd x build previously treated validation warnings (e.g. missing providers when the service-target-provider capability is declared) as command failures. azd x init then aborted on those warnings, and its build subprocess output was discarded so the user never saw why.

• ux.TaskList: a task returning (Warning, err) no longer cancels subsequent tasks, and warning details render with warning styling instead of error styling.
• azd x build reports validation warnings via the ux.Warning state and reserves the ux.Error state for missing required fields.
• validateExtensionMetadata is extracted as a shared helper so azd x init validates the in-memory schema directly. The old subprocess-output-parsing approach (ANSI stripping, marker probing, trailing-paren trimming) is gone.
• Validation warning messages now name the YAML field, the capability that requires it, and what to set.
• A short inline summary appears in the init task list; the full bulleted detail prints once below it.
• The redundant "Local extension source already exists." message is removed (the (-) Skipped task line already conveys that).

Wizard UX (resolves #8207)

• Confirmation prompt now defaults to "yes".
• Tags prompt is optional and accepts empty input. Also fixes a pre-existing bug where parsed tags were discarded and tags: was always empty.
• Dotted namespaces (ai.project) are translated into the actual command path (azd ai project <command>) when generating the usage field and the "Try out the extension" follow-up hint, matching how bindExtension registers nested namespaces. The namespace prompt help text now explains this behavior.
• Generated extension.yaml includes a # yaml-language-server: $schema=... modeline pointing at the published schema URL so editor tooling validates the file out of the box.

Scaffolded extension root command

The Go template produced Use: "azd {{namespace}} <command> [options]", which made cobra parse "azd" as the command name and rendered Usage: azd [command] in the extension's own --help output. The template now uses the leaf segment of the namespace for both Name and Use (matching the convention in azure.ai.agents), so azd ai test --help displays Usage: test <command> [options] as expected.