## Motivation and Context
#490 and #492 merged independently green but broke each other on main: #492 made the modern path reject
a name-bearing POST whose body carries a target name without `Mcp-Name` (-32020, HTTP 400)
and taught the `modern_rack_request` helper to derive `Mcp-Method` from the body, while #490,
merged in between, added five modern `tools/call` tests that build their requests through that helper
and therefore send no `Mcp-Name`. Since the #492 merge every CI run on main fails those five tests
with 400 where 200 is expected; the library behavior itself is correct.
The helper now derives `Mcp-Name` the same way it derives `Mcp-Method`: from `params.name` or
`params.uri` when the body's method is one of the name-bearing three. That is what a conforming client sends per
the 2026-07-28 specification, and what the TypeScript SDK's client, the bundled `MCP::Client::HTTP`,
and the conformance harness's standard headers all do, so tests built through
the helper model a compliant client by default. `name_header: nil` omits the header for tests of
the requirement itself, symmetric with `method_header: nil`, and an explicit `headers:` entry still overrides
the derived value, which keeps the base64 mismatch tests exercising their divergent names.
## How Has This Been Tested?
The five failing tests pass again without being touched, and the #492 requirement tests
(missing `Mcp-Method`, missing `Mcp-Name`, header mismatches, base64 decoding) still pass:
the transport test file reports 217 runs with zero failures. `bundle exec rake` is green,
including RuboCop and the `--requirements 2025-11-25` conformance baseline.
## Breaking Changes
None. The change is confined to a test helper; the shipped gem is untouched.
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