#3237 fix(dispatcher): advance mint counter past caller-supplied IDs (#3126)

Summary

• When a caller supplies an integer request ID, eagerly advance _next_id to max(_next_id, supplied_id) in both JsonRpcDispatcher and DirectDispatcher
• Prevents minted IDs from colliding with previously-completed caller-supplied IDs (spec requires IDs MUST NOT be reused within a session)
• Minimal 2-line fix per dispatcher — no behavioral change for string-only IDs

Test plan

• 4 new regression tests (2 per dispatcher via pair_factory parametrization)
• All 894 tests pass
• Covers both in-flight and completed caller-supplied ID scenarios

Fixes #3126