better support any using modern 1.25 ast and golang tools packages

[cce]

This updates #38 (already released in v1.1.64) to use Go 1.25's AST and package type parsing to detect any rather than checking if the identifier is any. It should produce the same code as #38.

[Copilot]

Pull request overview

This PR updates msgp’s parsing logic to use Go 1.25-era type information (via go/packages + go/types) to classify any and other interface-derived type declarations semantically, instead of relying on string checks, while also updating the repo/tooling to Go 1.25 and newer x/tools.

Changes:

• Switch AST/package loading to include type info and classify type declarations as interfaces based on go/types underlying types.
• Expand parser tests and add dedicated testdata to ensure any / interface aliases and named interface types are handled correctly.
• Bump Go/tooling versions and update CI to read the Go version from go.mod.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
go.mod	Bumps module Go version and updates golang.org/x/tools to support the newer go/packages usage.
go.sum	Refreshes sums for updated x/tools and transitive dependencies.
tests/go.mod	Bumps the nested test module’s Go version to match the updated toolchain baseline.
parse/getast.go	Uses packages type info (types.Info) to classify interface-like type declarations (including any and named/aliased interfaces).
parse/getast_test.go	Adds semantic/type-check-based tests for interface classification and a regression test using File() over testdata.
parse/testdata/anydecl/anydecl.go	Adds a concrete package fixture to exercise any/interface declarations through go/packages.
.github/workflows/build.yml	Updates CI to use go-version-file: go.mod rather than a hardcoded Go version.

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