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's validated path, repeated query, and ordered extension-option contract
• pace and recheck liveness before sending the confirmable empty-body request
• return the same (code, payload) shape as GET and POST
• retire the pending token after success, 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.

Stack / merge order

1. fix(protocol): pace CoAP request sends #51 owns request and OBSERVE-registration pacing.
2. feat(protocol): validate GET and POST request options #48 adds validated query and extension options.
3. feat(protocol): add CoAP DELETE requests #49 (this PR) adds DELETE.
4. feat(protocol): add bounded Block1 POST uploads #50 adds bounded Block1 POST uploads.

Please review this after #48. The DELETE-only commit is b808645.

Validation

• 308 SmartThings-Local tests against this exact head
• 1,625 LocalThings tests against this exact source tree
• focused DELETE encoding, pacing, timeout cleanup, closed-session, validation, and API-contract coverage
• bytecode compilation, diff checks, and share-safety checks
• Python 3.11 dependency-floor and package smoke checks at both endpoints of the stack

[QuiteYellow]

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