Post-GA: add structured missing-input metadata to the extension protocol

## Summary

Follow-up to #9571 and #9721. The PR keeps actionable missing-input remediation on the existing
`ActionableErrorDetail.suggestion` and `ExtensionError.suggestion` fields because the extension
protocol is approaching its GA schema freeze.

After GA, add explicit protobuf support so missing-input metadata can round-trip between azd hosts
and extensions without reducing the payload to rendered text.

## Deferred protocol changes

### `errors.proto`

- Add `InputSourceKind` values for flag, environment, and config sources.
- Add `InputSource` with the source name, example value, and complete executable example.
- Add `RequiredInput` with its description and supported sources.
- Add `PromptRequiredErrorDetail` with required inputs, the original prompt message, and the
user-facing error headline.
- Add an optional `prompt_required_error` field to:
- `ActionableErrorDetail` for host-to-extension errors.
- `ExtensionError` for extension-to-host errors.

### `prompt.proto`

- Add an optional `prompt_required_error` field to `ConfirmOptions`, `PromptOptions`, and
`SelectOptions` so extensions can describe remediation before invoking a generic prompt.
- Keep `MultiSelectOptions` unchanged unless its no-prompt behavior is changed to require input.

### Generated and template surfaces

- Regenerate the Go messages in `cli/azd/pkg/azdext`.
- Keep the protocol templates under
`cli/azd/extensions/microsoft.azd.extensions/internal/resources/languages/proto/` aligned with
the core definitions.
- Update generated language SDKs and extension SDK reference documentation.

## Compatibility and rollout

- Make the schema changes additive and preserve all GA field numbers and semantics.
- Retain the rendered `suggestion` fallback so old hosts and extensions remain actionable.
- Add version-skew coverage for old host/new extension and new host/old extension combinations.
- Remove the need for extensions to inspect unknown protobuf fields with `protowire`.

## Acceptance criteria

- `input.PromptRequiredError` metadata round-trips in both host-to-extension and
extension-to-host directions.
- Generic confirm, prompt, and select RPCs preserve supplied missing-input metadata when prompts
are disabled and no default satisfies the request.
- Flag, environment, and config sources retain complete executable examples.
- Core and language-template proto definitions remain synchronized.
- Generated SDKs, extension documentation, and regression tests are updated together.
- Existing suggestion-only peers continue to receive useful remediation.

## Timing

Do not land these protocol changes until the GA schema freeze has been lifted.