fix: preserve explicit notification preference false

[pijiang3]

What changed

• Regenerate notification override request fields as *bool, preserving explicit false on the wire.
• Cover incident create, responder add, and incident assign requests.
• Add the previously missing assignment notification model.
• Add request serialization and generator regression tests.
• Include the reviewed implementation design.

Root cause

FollowPreference was generated as bool with omitempty, so setting it to false removed follow_preference from JSON. The backend only honors personal_channels when the field is explicitly present and false.

Developer impact

This is a public Go type change. Existing keyed literals must change from FollowPreference: false to FollowPreference: flashduty.Bool(false). Publish as v0.13.0, not a patch release.

Validation

• make check
• golangci-lint: 0 issues
• go test -race ./...
• go build ./...
• generation drift/determinism check
• explicit-false and unset-field wire regression tests