MCP server headers: unresolved `$VAR` placeholders are sent verbatim instead of failing closed

### This issue is for a: (mark with an `x`)
```
- [x] bug report -> please search issues before submitting
- [ ] feature request
- [ ] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Summary

`discovery/mcp.py` validates unresolved `$VAR` / `%VAR%` placeholders on an MCP server's `url` and on `auth.client_id`, but **never on header values**. A typo'd or missing app setting therefore ships the placeholder text to the remote server verbatim, with no warning.

This affects the **default `in_lang_worker` backend today** — it is not specific to `session_runtime.aca_sandbox`.

### Minimal steps to reproduce

1. Add an `mcp.json` with a header referencing an environment variable that is **not** set:

```json
{
"servers": {
"example": {
"type": "http",
"url": "https://api.example.com/mcp",
"headers": { "Authorization": "Bearer $MY_TOKEN_TYPO" }
}
}
}
```

2. Start the app without defining `MY_TOKEN_TYPO`.
3. Invoke an agent that uses the `example` MCP server.

### Any log messages given by the failure

**None — that is the bug.** No warning, no skipped-server message. The server loads normally and the request goes out with the literal header value.

Contrast with the existing, correct handling for the other two fields:

- `discovery/mcp.py:134` — `if has_unresolved_placeholders(url): ... skipping`
- `discovery/mcp.py:67` — `if has_unresolved_placeholders(client_id): client_id = ""`

`_build_header_provider` (`discovery/mcp.py:37-53`) applies no such check; it stringifies whatever is present:

```python
static_headers = (
{str(key): str(value) for key, value in headers.items()}
if isinstance(headers, dict)
else {}
)
```

### Why the value is a literal rather than empty

`config/env.py:29-34` returns the original match when the variable is unset:

```python
def _dollar_replacer(match: re.Match[str]) -> str:
return os.environ.get(match.group(1), match.group(0))
```

This is documented behavior (`docs/front-matter-spec.md:928`): *"If a referenced environment variable is not set, the original placeholder text is left literal."* So the outbound header is exactly `Authorization: Bearer $MY_TOKEN_TYPO`.

### Expected/desired behavior

Fail closed and loudly, consistent with how `url` and `auth.client_id` are already treated. Either:

- skip the server with a warning naming the server and the unresolved variable (matches the `url` precedent), or
- fail startup.

The `url` precedent seems the closer match.

### Secondary concern: information disclosure

Because the placeholder is sent verbatim, the **name of the app setting** is disclosed to the remote MCP host on every request. Minor, but it is unnecessary and avoidable, and it will typically end up in the remote's request logs.

### Versions

Observed on `feature/aca-sandboxes` @ `343e3e0`; the affected code is not branch-specific and applies to `main`.

### Mention any other details that might be useful

- The helper needed already exists and is already imported in this module — `has_unresolved_placeholders` from `config.env`. The fix is a check in `_build_header_provider`, plus a regression test.
- Worth deciding whether the check applies to **header names** as well as values (dictionary keys are never substituted, per `docs/front-matter-spec.md:931`, so a `$VAR`-shaped key is always literal and is probably an authoring mistake worth warning about).
- Found while investigating secret handling under the ACA sandbox backend, but it is independent of that work and worth fixing on its own merits.

Start in discovery/mcp.py, especially _build_header_provider and the existing unresolved-placeholder checks for url and auth.client_id. Review config/env.py and the documented substitution behavior, then find the MCP discovery tests to reproduce an unset variable in a header. Done means unresolved header placeholders are handled consistently with the url precedent, with warning or skip behavior covered by a regression test.

Written by the indexing model from the issue text.