## Motivation and Context
Replace server-only `http.MaxBytesReader(nil, ...)` with
`io.LimitReader` to cap the fetched body to 4KB in key fetcher. This
avoids passing a `nil` ResponseWriter and aligns with Go idioms. If more
than 4KB is received, return a clear error. Previously `MaxBytesReader`
returned a misleading `http: request body too large` error, which is
server-oriented. With this change the error becomes `HTTP auth key
response too large`.
## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->
Add tests covering success, oversized bodies, connection failure,
non-200 status, and read failures. This covers almost all lines of
`FetchKey` method of `DefaultHTTPKeyFetcher`. For tests, I also added a
new initialiser `NewDefaultHTTPKeyFetcherWithClient` so that the HTTP
client could be modified for test use.
Tests use `httptest.NewTLSServer` plus a helper to construct a client
pinned to the test server.
`auth` package test coverage increased from 56.8% to 59.3%.
## Breaking Changes
<!-- Will users need to update their code or configurations? -->
None.
## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [ ] I have added or updated documentation as needed
## Additional context
<!-- Add any other context, implementation notes, or design decisions
-->
Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>
76365a61e3