Preserve actionable remediation in fail-fast errors for agent workflows

## Summary

Follow-up to #9570. This issue intentionally excludes the prompt-equivalence, destructive-confirmation, manifest-parameter, eval-context, init-overwrite, and ACR-selection gaps already tracked there.

For agent and CI workflows, fail-fast errors should be self-remediating: name the missing input, list every supported flag/environment/config source, and include an executable example. The remaining gaps below cause coding agents to guess, retry, and consume tokens without making progress.

## Findings

| Priority | Area | Finding |
| --- | --- | --- |
| High | Prompt RPC boundary | `input.PromptRequiredError` already models missing inputs and flag/environment/config sources, but generic extension prompt RPCs create only `PromptMessage`. Across gRPC this degrades to `prompt required`; `azure.ai.agents` then wraps the raw error in `FromPrompt`. The rich input metadata and remediation never reach the extension/user. Also, `PromptRequiredError.ToString` generates executable examples only for environment sources, not flag or config sources. See `cli/azd/pkg/input/prompt_required_error.go`, `cli/azd/internal/grpcserver/prompt_service.go:56-60,95-99,207-210`, and `cli/azd/extensions/azure.ai.agents/internal/exterrors/errors.go:253-276`. |
| High | Environment selection | Several extension lifecycle paths return `no current azd environment is selected`, `current azd environment has no name`, or only suggest `azd env new`. They should name all actual sources: `-e/--environment`, `AZD_ENVIRONMENT`, and `azd env select `, with an example such as `azd -e dev ...`. See `azure.ai.agents/internal/cmd/activity_bot_provision.go:27-32`, `azure.ai.projects/internal/cmd/project_service_config.go:45-55`, and `azure.ai.projects/internal/provisioning/foundry_provisioning_provider.go:641-651`. |
| High | Eval generation | The fail-fast required-input error says one of `--gen-instruction`, `--gen-instruction-file`, `--config`, or dataset/evaluator inputs is required, but `eval generate` does not expose a `--config` flag. It also combines multiple alternatives into one plain error without a complete example command. See `azure.ai.agents/internal/cmd/eval_generate.go:82-94,203-207`. |
| Medium | Azure dependency values | Agent deployment errors correctly name `AZURE_SUBSCRIPTION_ID`, `AZURE_LOCATION`, and `FOUNDRY_PROJECT_ENDPOINT`, but remediation is indirect (`run provision`, `run init`, or `--project-id ...`) instead of giving the exact supported command, such as `azd env set AZURE_LOCATION eastus2`. See `azure.ai.agents/internal/project/service_target_agent.go:301-307,528-533,1873-1878,2613-2618`. The Foundry provider subscription/location errors are the stronger pattern to reuse. |
| Medium | Deployed-agent state | `show`, `monitor`, `files`, and `code download` report that agent name/version could not be resolved, then say to deploy or “check environment values” without printing the expected `AGENT__NAME` / `AGENT__VERSION` keys or a service-specific command. `code download` additionally says to pass the agent name positionally even though its positional argument is a service name. See `azure.ai.agents/internal/cmd/show.go:83-95`, `monitor.go:93-99`, `files.go:85-90`, and `code.go:90-96`. |
| Policy | Plain or empty remediation | Multiple user-fixable paths use plain `fmt.Errorf`, so no structured suggestion reaches the azd host. Several `exterrors.Dependency` calls also pass an empty suggestion, including project initialization/not-found paths. User-fixable `Validation`/`Dependency` errors should not be emitted without remediation. |

## Desired error shape

```text
error: azd environment name is required

Provide one of:
Flag: --environment (or -e )
Environment: AZD_ENVIRONMENT
Config: current environment selection

Examples:
azd ai agent show -e dev
$env:AZD_ENVIRONMENT = "dev" # PowerShell
azd env select dev
```

The concrete sources and examples should reflect what the command actually reads. In particular, the azd environment selector variable is `AZD_ENVIRONMENT`, not `AZURE_ENV_NAME`.

## Acceptance criteria

- Preserve missing-input metadata across the host/extension gRPC boundary.
- Render copy-paste examples for flag, environment, and config input sources.
- Add a shared structured missing-input helper for extensions instead of plain `fmt.Errorf`.
- Environment-selection failures name `--environment/-e`, `AZD_ENVIRONMENT`, and `azd env select`.
- Remove the nonexistent `--config` remediation from `eval generate` or implement the flag.
- Missing Azure values include exact `azd env set ` commands where manual setting is supported.
- Missing deployed-agent state identifies the expected environment keys and the exact service-targeted deployment command.
- Add regression tests that assert the rendered error contains the input name, supported sources, and an executable example.