Remove GetClaimsFromContext backward compatibility helper

[JAORMX]

Summary

Remove GetClaimsFromContext function and migrate all usages to IdentityFromContext pattern following the authentication unification completed in #2437.

Changes

• Remove GetClaimsFromContext() from pkg/auth/context.go
• Update all test files to use IdentityFromContext() and access identity.Claims directly when needed
• Remove dedicated GetClaimsFromContext test functions
• Remove unused jwt imports from test files

Rationale

GetClaimsFromContext was added as a backward-compatibility helper during the Identity struct unification in #2437. All production code has migrated to using IdentityFromContext directly, with zero production usages remaining. Tests should verify the actual production API contract.

The function's comment even stated: "This is a helper function for backward compatibility with code that expects MapClaims. New code should use IdentityFromContext and access the Claims field directly."

Since all code is now "new code" using IdentityFromContext, the backward compatibility layer is no longer needed.

Impact

• Removes ~130 lines of code (function + tests)
• Simplifies the auth API to a single consistent pattern
• Improves test clarity by testing the actual production contract
• All tests pass - no functional changes to production code

Production Usage Analysis

Verified that GetClaimsFromContext had zero production usages:

• pkg/audit/auditor.go uses IdentityFromContext()
• pkg/authz/cedar.go uses IdentityFromContext()
• pkg/auth/tokenexchange/middleware.go uses IdentityFromContext()

The only usages were in test files, which have been updated to use the production pattern.

Testing

All tests pass:

go test ./pkg/auth/...
go test ./pkg/audit/...
go test ./pkg/authz/...

Linting clean:

task lint-fix