fix(sdk): enforce workspace policies in toolkits

[donmasakayan]

Summary

• Combine provider policy results with workspace policy rows for active provider executors.
• Preserve the most restrictive action, so workspace require_approval and block rules cannot be bypassed by a toolkit-scoped executor.
• Apply the same composition to tools listing, schema lookup, policy resolution, and execution.
• Add regression tests for provider and prepared-provider paths, including a real toolkit-scoped executor.

This addresses issue 1752. The implementation follows the least-privilege behavior proposed in the earlier PR 1846 by Adityakk9031, while excluding that PRs unrelated React change.

Validation

• bun run test -- src/policies.test.ts (43 passed)
• bun run test -- src/server.test.ts (6 passed)
• root lint passed
• root typecheck passed
• formatting check passed

No application-specific policy semantics were changed for executors without an active policy provider.