fix(mcp): Fix sandbox read-only mode not being applied to MCP sessions

[harsh21234i]

Summary

This fixes a header mismatch between the task sandbox and the MCP service
that was preventing read-only mode from being enforced for sandbox-created
MCP sessions.

The sandbox was sending x-posthog-read-only, while the MCP service was
only looking for x-posthog-readonly. Because of that mismatch, sandbox
sessions could silently miss the read-only restriction even when they were
meant to be locked down.

What changed

• updated the sandbox MCP config builder to send the canonical x-posthog-
  readonly header
• updated the existing Python tests to assert the correct header name
• added a small compatibility fallback on the MCP side so it still accepts
  the old x-posthog-read-only header
• added MCP-side regression coverage for both header spellings, plus the
  existing query-param fallback

Why

The main fix is making the sender and receiver agree on the same header
name. I also kept the MCP side backward-compatible so older internal
callers or in-flight clients don’t silently lose read-only enforcement
while everything catches up.

Validation

I verified the touched Python files compile cleanly locally. I didn’t run
the MCP vitest suite from this machine, so CI will be the final check.

Fixes #57268

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
services/mcp/tests/unit/readonly-header.test.ts:5-27
**Prefer parameterised tests**

All three cases test the same function with different inputs and the same expected output — this is exactly the pattern `it.each` is designed for. The Python side of this very PR uses `@parameterized.expand` correctly; the TypeScript side should follow the same convention.

```suggestion
    it.each([
        ['canonical x-posthog-readonly header', { 'x-posthog-readonly': 'true' }, ''],
        ['legacy x-posthog-read-only header', { 'x-posthog-read-only': '1' }, ''],
        ['readonly query parameter', {}, '?readonly=true'],
    ])('reads read-only from %s', (_, headers, qs) => {
        const request = new Request(`https://mcp.posthog.com/mcp${qs}`, { headers })
        expect(getReadOnlyFromRequest(request, new URL(request.url))).toBe(true)
    })
```

_{Reviews (1): Last reviewed commit: "fix(mcp): align sandbox readonly header" | Re-trigger Greptile}

[harsh21234i]

hey @haacked have you checked this?