feat(schemas): add media_buy.frequency_capping capability declaration (closes #4640)

[bokelley]

Summary

Adds an additive media_buy.frequency_capping capability block to get_adcp_capabilities so sellers can declare frequency-cap support up front and buyers can pre-flight validate before submitting.

Shape (B-lite per WG triage on #4640):

• Presence of the object = seller honors targeting.frequency_cap on packages and MUST reject caps it cannot enforce (no silent drops).
• supported_per_units (optional) — entity granularities, $ref to /schemas/enums/reach-unit.json.
• supported_window_units (optional) — duration units (hours, days, campaign, etc.) aligned with duration.json's unit enum.

Both sub-fields are optional; omission means "all values supported." Neither sub-field is required on the parent object.

Why

• Today there's no capability declaration for frequency capping. Sellers either honor targeting.frequency_cap silently or drop unenforceable caps without telling the buyer — a façade-prevention gap.
• Two aligned arrays let buyer agents short-circuit submission for unsupported reach-unit / window combinations rather than discovering at activation.
• Façade prevention: a seller advertising the block but rejecting all caps gets caught by the capability-claim contract (Meta: capability-claim contract scenarios — close the gap between what agents declare and what they do #4637) once the corresponding frequency_cap_enforcement storyboard lands.

What was dropped

The original RFC included enforces_within (a duration attestation of "we enforce caps within X minutes of impression"). Dropped per WG triage on #4640 — no SSP can back that attestation cleanly across DSP/SSP/exchange layers, so the field would have invited unfalsifiable claims.

Per-product overrides

Sellers with mixed addressable / non-addressable inventory may need per-product overrides on targeting_capabilities. Out of scope here — file a follow-up RFC if needed. The seller-level declaration is the floor.

Notes

• supported_window_units items shape: {type: string} (free string with documented enum values). duration.json defines the unit enum inline on the object schema rather than as a standalone enum file, so there's no $ref target. The description names the canonical values (hours, days, campaign).
• The corresponding frequency_cap_enforcement storyboard scenario lands in a separate PR under the capability-claim contract pattern (Meta: capability-claim contract scenarios — close the gap between what agents declare and what they do #4637).
• Build: npm run build:schemas and npm run build:compliance both pass.

Refs

• Closes RFC: frequency_capping capability declaration — what does a seller need to commit to? #4640 (RFC)
• Pattern: Meta: capability-claim contract scenarios — close the gap between what agents declare and what they do #4637 (capability-claim contract / capability-gated scenarios)

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