refactor(mix_protocol): export breakpoint bounds as oneOf

[leoafarias]

Description

_BreakpointBoundsConstraint.toJsonSchema() spelled its either/or as if/then/else wrapped around a not. This replaces it with oneOf:

'oneOf': [
  {'required': ['token']},
  hasBounds,
]

Identical truth table — token alone matches one branch, a width bound alone matches one branch, both match two, neither matches zero — which is exactly what isValid enforces.

Why

The if/then/else form needed a nested negation and a second copy of the hasBounds block per site. The oneOf form needs neither. Net effect on the checked-in goldens:

	before	after
if / then / else / not	23 each (style), 53 each (chart)	0
oneOf	0	23 / 53
anyOf (style)	302	279

The anyOf drop is exactly the 23 duplicated hasBounds blocks. Baseline had zero oneOf, so nothing was clobbered by the new key.

Verification

• Every accepted and rejected document is unchanged. tool/schema-check validates all 284 fixtures across the four suites with Ajv: 284/284, same verdicts as before.
• mix_protocol 404 tests, mix_chart_protocol 15, mix_winds 824 — green. dart analyze clean on all three, dart format clean.
• Goldens regenerate to exactly what is checked in (melos run schema:fixtures produces no further diff).

mix_chart_protocol is regenerated too, since it shares the codec and its checked-in schema had 53 stale if sites.

One thing that did not change

Ajv still needs strictRequired: false. Its lint does not resolve required against the parent's properties through any applicator branch, so oneOf trips it exactly as if did — I tested enabling it and all three suites still fail to compile. The option stays; its comment now describes the current spelling rather than the old one.