feat(protocol): add CoAP DELETE requests

[Jason-Morcos]

Summary

• add METHOD_DELETE and a public DtlsCoapSession.delete() request primitive
• reuse feat(protocol): validate GET and POST request options #48 validation for paths, repeated queries, and ordered extension options
• pace and recheck liveness before sending the confirmable empty-body request
• register the request in the shared token/MID exchange registry and wait through _wait_live
• return the same (code, payload) shape as GET and POST
• clean both pending indices after response, reset, timeout, or reader failure

Scope

This is the second PY-09 slice. It adds the wire-level method only: resource selection, authorization policy, ownership transactions, response interpretation, and Block1 behavior are unchanged.

The implementation is restacked on the current shared-MID and #54 liveness substrate. A bare RST is matched by MID, a response dispatched just before reader teardown still wins, and no path returns with a stale token or MID registration.

The DELETE-only commit is 11391db.

Stack / merge order

1. feat(protocol): validate GET and POST request options #48: validated query and extension options
2. feat(protocol): add CoAP DELETE requests #49 (this PR): DELETE
3. feat(protocol): add bounded Block1 POST uploads #50: bounded Block1 POST uploads

Merge #49 after #48. It no longer depends on the old #51 branch.

Validation

• 465 SmartThings-Local tests against this exact head
• all 1,730 LocalThings tests at b5e25d7 against this exact source tree
• focused DELETE encoding, pacing, RST, timeout cleanup, reader-death, response/teardown race, validation, and API-contract coverage
• current Home Assistant request behavior compared against the working integration; its 55 focused transport/resilience tests pass
• bytecode compilation, diff checks, and share-safety checks
• all eight GitHub jobs pass on this exact head, including Python 3.11-3.14, dependency-floor/latest, package, and share safety

[QuiteYellow]

Not reviewed yet, same as #48. #56 holds the order.