http: invalidate cached token scopes when auth token changes

[davidahmann]

Problem

If scope context populated for one token is reused after a token change, tool filtering can reflect stale permissions.

Why now

Token rotation/swap during long-lived request flows is common; scope filtering must stay bound to the active token to avoid quiet governance drift.

What changed

• Added token-bound scope context helpers:
  • WithTokenScopesForToken
  • GetTokenScopesForToken
• Updated middleware/filtering to reuse scopes only when bound token matches active token:
  • pkg/http/middleware/pat_scope.go
  • pkg/http/middleware/scope_challenge.go
  • pkg/http/handler.go (PATScopeFilter)
• Added regression tests:
  • pkg/context/token_test.go
  • token swap refetch case in pkg/http/middleware/pat_scope_test.go

Validation

• go test ./pkg/context ./pkg/http/middleware ./pkg/http

Refs #2203

[davidahmann]

This PR prevents stale scope-filter decisions when auth tokens change by requiring cached scope context to match the active token before reuse.

The fix is intentionally small: add token-bound scope helpers, switch PAT/scope-challenge/filter paths to token-bound reads, and add regression tests for token-swap behavior.

Validation:

• go test ./pkg/context ./pkg/http/middleware ./pkg/http (pass)

Inspired by research context: CAISI publishes independent, reproducible AI agent governance research: https://caisi.dev