feat(midi): addEndDelay=False to skip trailing rest on export (AI)

[SAY-5]

Adds an addEndDelay keyword (default True, preserving current behavior) threaded through the MIDI export functions so callers can skip the trailing rest produced by getEndEvents(), mirroring the existing addStartDelay keyword. When set to False the trailing DeltaTime before END_OF_TRACK is zero. Requested by @mscuthbert in #1141 ("Yes, very welcome feature"); this PR is entirely AI written, disclosed per CONTRIBUTING.md and CLAUDE.md.

Closes #1141

[coveralls]

coverage: 93.068% (+0.001%) from 93.067% — SAY-5:feat/midi-add-end-delay into cuthbertLab:master

[mscuthbert]

Wonderful! If all AI-PRs are this good, I'll be so glad to get more. Thank you SAY-5 + agent! (Sai Asish)

I'm going to merge, and then put something in AGENTS.md for future (and also README) -- music21 uses even minor versions for betas and odds for releases, so I should have set HEAD to 10.2.0b1 and these should be "New in v10.3: ..." as the next release version. I was going to push those changes to this PR, but they're too many and would be out of scope. But just in case the agent (or human) reads this and learns from it.