fix(schema): make account.supported_billing conditional on media_buy protocol

[bokelley]

Closes #3746

Summary

The get_adcp_capabilities response schema required account.supported_billing unconditionally for all agents, even those that don't declare media_buy in supported_protocols. The field has no semantics outside of media buying (operator-pays / advertiser-pays arrangements), and the schema's own description already stated "Expected when media_buy is in supported_protocols." This mismatch caused every non-media-buy agent (signals, creative, governance, brand, SI, measurement) that included an account block to fail schema validation on a single irrelevant field, which cascades into SDK v2-downgrade loops and storyboard failures across the board.

Changes:

1. Removes "required": ["supported_billing"] from the account object definition — it was enforced unconditionally on any agent that included an account block.
2. Adds a root-level allOf if/then constraint: account and account.supported_billing are required only when supported_protocols contains "media_buy". Uses the same contains: { const: "..." } + required: [...] guard pattern as governance/sync-plans-request.json.
3. Updates account.description to surface the conditional rule at first read.
4. Updates media_buy.description from "should include" to "MUST include" — the if/then now normatively enforces it.

Non-breaking justification: Relaxes a constraint for non-media-buy agents (they may now omit supported_billing from an account block without failing validation). Preserves the constraint for media-buy agents via the then branch. Any conformant 3.0.0 media-buy implementation already satisfies the new MUST — the description established the expectation; the schema constraint now aligns with it.

Code-generator note: draft-07 if/then is dropped by openapi-typescript, zod-to-json-schema, datamodel-code-generator pre-0.25, and quicktype. SDK generator targets should add a runtime guard requiring supported_billing when media_buy is declared and account is present. (The same caveat already applies to adcp.idempotency's oneOf in this schema per its existing inline comment.)

Pre-PR review

• code-reviewer: approved — JSON Schema draft-07 valid, allOf placement correct, then.properties.account.required correctly overlays root account definition; redundant required: ["supported_protocols"] guard inside if is harmless (root-level required already enforces it); patch changeset level correct.
• ad-tech-protocol-expert: approved — non-breaking per spec; then.properties.account type guard technically redundant given root-level "type": "object" on account (and the established repo pattern in sync-plans-request.json doesn't add type guards either); description nits addressed in this diff.

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See #3121
for context.

Session: https://claude.ai/code/session_01VE9kbAjX6nAmAruFXbUeka

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