…arded Host header (#26656)

## Problem

PR #23109 introduced port normalization for the private IP blocking
feature, which mutated `CoderAccessURL.Host` to always include the
default port (e.g. `coder.example.com:443`). This leaked into the `Host`
header of every request forwarded to the Coder server.

When `CODER_REDIRECT_TO_ACCESS_URL=true`, the `redirectToAccessURL`
middleware compared the `Host` header literally against the access URL
(`coder.example.com`), saw a mismatch, and returned a 307 redirect to
the Coder dashboard HTML page.

Copilot then received HTML instead of JSON:

```
Failed to start MCP client: Streamable HTTP error: Unexpected content type: text/html; charset=utf-8
Failed to load custom agents: SyntaxError: Unexpected token '<', "<!doctype "... is not valid JSON
```

## Changes

- Stop mutating `coderAccessURL.Host`; store the resolved port in a
separate field for `isBlockedIP`
- Update existing tests that asserted the old (mutated) `.Port()`
behavior
- Add test cases verifying the Host is preserved with and without an
explicit port

> Generated with the assistance of Coder Agents on behalf of
@ssncferreira

(cherry picked from commit c41d219)