Reduce crypto/internal/backend/fips140 API surface

[qmuntal]

crypto/internal/backend/fips140 is intended to be used from upstream crypto packages, but there are some APIs that are specific to crypto/internal/backend, which is not part of upstream.

Moving these API to unexported crypto/internal/backend APIs will make future refactors easier.

[Copilot]

Pull request overview

This PR refactors the crypto/internal/backend/fips140 integration to reduce backend-specific surface area by moving FIPS validation logic into crypto/internal/backend (via a new unexported checkFIPS helper) and updating backend implementations to use it.

Changes:

• Adds crypto/internal/backend/checkFIPS and rewires backend init-time FIPS validation to call it instead of fips140.Check.
• Moves requirefips / ms_skipfipscheck build-tag plumbing from crypto/internal/backend/fips140 into crypto/internal/backend.
• Adjusts crypto/internal/backend/fips140 to expose state needed by checkFIPS (Message, Require) and removes the previous Check API.

Show a summary per file

File	Description
patches/0002-Add-crypto-backends.patch	Updates the crypto-backends patch to relocate FIPS-checking logic into crypto/internal/backend and revise the fips140 package API accordingly.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 3