Backend MCP instructions are replaced by the proxy's own

### What happens

The proxy returns its own `instructions` on `initialize`, discarding whatever the backend sent. A client is told about the transport instead of about the tools.

Measured against a Bedrock AgentCore Gateway on v1.6.6:

| | `serverInfo` | `instructions` |
| --- | --- | --- |
| Backend, called directly with SigV4 | `mtp-mercell-mcp` | `Tools over ... development and operational data. All tools are read-only ... Call skills_get first for guidance ...` |
| Through `mcp-proxy-for-aws` | `MCP Proxy for AWS` | `MCP Proxy for AWS provides access to SigV4 protected MCP servers through a single interface. ...` |

### Why it matters

`instructions` is the one channel a backend has to tell a client what it is for, and it is delivered only on `initialize`. Our gateway uses it to point clients at a tool that loads platform guidance. That pointer never arrives, so the guidance is never loaded — and there is no way to tell from the client side that anything was dropped.

Tool names, descriptions and schemas all come through unchanged, so those remain a usable channel. `instructions` is the only one that does not.

### Where it comes from

`InitializeMiddleware._overwrite_init_options` already reads the backend's `InitializeResult` and copies one field out of it:

```python
fastmcp_ctx._session._init_options.capabilities = init_result.capabilities
```

Its docstring says the response will contain "the backend server's info instead of the proxy's defaults", but only `capabilities` is copied. `instructions` is a field on `InitializationOptions` already, so the seam exists.

### Suggested behaviour

Opt-in, so nothing changes for clients relying on the current text:

- `--proxy-instructions`, or `AWS_MCP_PROXY_INSTRUCTIONS=1`
- applied only when the backend actually sends instructions, so a backend without any keeps the proxy description rather than an empty field
- `serverInfo` left alone — there genuinely is a proxy in the path

Happy to open a PR; I have this working and tested locally.

### Environment

`mcp-proxy-for-aws` 1.6.6, Python 3.12, backend is a Bedrock AgentCore Gateway (`--service bedrock-agentcore`).

Start at InitializeMiddleware._overwrite_init_options and its docstring, then inspect how initialization options and proxy configuration are handled. Add coverage for the opt-in behavior where backend instructions are preserved only when supplied, and verify that the proxy fallback remains unchanged when they are absent.

Written by the indexing model from the issue text.