Misc cleanup

[philljj]

Description

• Fix error return in esp_check_icv_hmac (return -1 instead of num bytes differing).
• Refactor ip header update to esp_transport_wrap (removes duplicated code).
• Clean up esp tests so they correctly return error if built without certain alg support.

[Copilot]

Pull request overview

This PR performs small ESP-related cleanups to make error returns consistent, centralize IP-header fixups in the wrap path, and ensure ESP test binaries fail fast when required crypto support is not compiled in.

Changes:

• Normalize HMAC ICV check failure return to -1 instead of a non-zero compare value.
• Move IPv4 header updates (len/proto/checksum) into esp_transport_wrap() and remove duplicate fixups in unit tests / send path.
• Update ESP integration tests to return an error when built without GCM-stream or DES3 support, and avoid initializing the stack if the selected mode cannot be supported.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/wolfesp.c	Normalizes ICV mismatch return and refactors IP header updates into esp_transport_wrap() (removing duplication in esp_send()).
src/test/unit/unit_esp.c	Removes manual proto/len/checksum fixups that are now handled by esp_transport_wrap().
src/test/esp/test_esp.c	Fails early with an error when selected ESP mode requires missing algorithm support; defers stack init until after SA setup succeeds.
src/test/esp/esp_server.c	Same “fail early if alg not built” behavior for the server test binary.
Makefile	Clarifies debug-flag comments.

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