.github/workflows: add nightly conformance tests

[findleyr]

Add a conformance test job to the nightly GitHub workflow that runs the SDK conformance tests using ./scripts/conformance.sh.

The job:

• Runs on ubuntu-latest
• Sets up Go 1.25
• Sets up Node.js 20 (required for npx)
• Executes the conformance test script

This ensures the SDK is regularly tested against the official MCP conformance test suite to catch regressions early.

PR Guideline

Typically, PRs should consist of a single commit, and so should generally follow
the rules for Go commit messages.

You must follow the form:

net/http: handle foo when bar

[longer description here in the body]

Fixes #12345

Notably, for the subject (the first line of description):

• the name of the package affected by the change goes before the colon
• the part after the colon uses the verb tense + phrase that completes the blank in, “this change modifies this package to ___________”
• the verb after the colon is lowercase
• there is no trailing period
• it should be kept as short as possible

Additionally:

• Markdown is allowed.
• For a pervasive change, use "all" in the title instead of a package name.
• The PR description should provide context (why this change?) and describe the changes
  at a high level. Changes that are obvious from the diffs don't need to be mentioned.

[maciej-kisiel]

IIUC this would require to be merged into a branch to actually run the workflow to test it, so it's probably not worth the effort. In that case, it looks good to me.