mcp: check out testdata fixtures with LF line endings

[PerryLink]

On Windows a default git clone uses core.autocrlf=true, so mcp/testdata/files/info.txt lands on disk with CRLF. TestServerConformance asserts the exact base64 of that file, so the test fails on every such checkout:

--- FAIL: TestServerConformance/resources.txtar
        -               "blob": "Q29udGVudHMK",
        +               "blob": "Q29udGVudHMNCg==",
FAIL	github.com/modelcontextprotocol/go-sdk/mcp

Decoded, those are "Contents\n" and "Contents\r\n" respectively -- the only difference is the checkout, not the content.

A 6-line .gitattributes pins the fixtures (and all *.txtar) to LF, which is what the assertions assume:

git ls-files --eol mcp/testdata/files/info.txt
  without .gitattributes:  i/lf  w/crlf  attr/                -> go test ./mcp/ -run TestServerConformance   FAIL
  with    .gitattributes:  i/lf  w/lf    attr/text eol=lf     -> go test ./mcp/ -run TestServerConformance   ok

go test ./... passes with the change.

Caveat for anyone with an existing Windows clone: the fixture is already on disk as CRLF, so it needs git checkout -- mcp/testdata/files/info.txt (or a fresh clone) before the fix takes effect locally.