feat(negotiation): add first-class buyer and seller proposal APIs

[bokelley]

Context

Add first-class proposal-negotiation APIs to the Java/Kotlin client, server, and testing modules for AdCP 3.2 (#6450, adcontextprotocol/adcp#6547).

Parent program: adcontextprotocol/adcp#6556

Acceptance criteria

Client

• Generated/maintained sealed or otherwise safely discriminated models for refine_proposals outcomes.
• Capability-aware builders that enforce supported dimensions, seller ceilings, and protocol cardinality before transport.
• Verification utilities for response ordering, budget and product constraints, unique alternatives, machine-readable failure subsets, and finalize/expiry semantics.
• Reactive and Kotlin surfaces preserve the same error-first and outcome-safe behavior.

Server

• Handler registration and capability declaration for proposal refinement.
• Batch preflight before mutation, with commercial decisions delegated to application callbacks.
• Hooks/utilities for immutable successors, digest/lineage, validated persistence, and atomic finalize transactions.

Testing and examples

• Extend adcp-testing with fixtures/assertions for all outcomes, reason codes, mixed-batch rejection, 10/25 limits, fail-closed budgets, partial alternatives, idempotency, and finalization atomicity.
• Add a runnable buyer/server example compatible with the training seller scenario in epic: make proposal negotiation easy to implement, teach, and verify adcp#6556.
• Document legacy refinement compatibility limits.

Dependency

Blocked on the final 3.2 schema/API shape from adcontextprotocol/adcp#6547.

Out of scope

• Commercial pricing and optimization policy.

[bokelley]

The protocol surface grew in adcontextprotocol/adcp#6547 (efe48f522e) after a pre-merge red team. Delta to pick up on top of the original acceptance criteria:

• Three new typed constraints alongside total_budget: cpm {max, currency} (every purchase priced fixed cpm/vcpm in currency at or under max), impressions {min} (summed purchase impressions), flight {start_no_later_than?, end_no_earlier_than?} (checked against envelope start_time/end_time; asap never satisfies a start bound). Builders, validators, and capability gating should cover all of them.
• Dimension rename: the capability value is product_changes (was product_selection in the draft); dimensions now match request field names exactly.
• unsatisfied_constraints is open strings, not a closed enum — parse it as constraint keys echoed from the request, and don't bake an enum into generated types.
• New reason codes hold_unavailable and batch_aborted, plus a precedence rule worth encoding in verification helpers: constraint_unsatisfiable wins whenever a typed constraint failed; typed failures never surface as commercially_declined. Double-finalize of a held draft is task-level INVALID_STATE.
• Lineage: every returned proposal must carry parent_proposal_id equal to the entry's source_proposal_id — a cheap, high-value response-verification check.
• Digest verification is now normative: terms_digest = sha256: + base64url(SHA-256(RFC 8785 JCS of commercial_terms)). Buyer helpers should recompute and compare rather than trusting the string, and treat alternative distinctness as distinct commercial_terms.
• Partial invariant: on partial, every draft satisfies every constraint absent from unsatisfied_constraints — verification helpers can and should assert this.
• UNSUPPORTED_FEATURE details now follow error-details/unsupported-refinement-dimension.json (unsupported_dimension + echoed supported_dimensions) — useful for typed error recovery.

Java-specific note: digest verification needs RFC 8785 canonicalization — org.webpki.jcs (json-canonicalization) is the reference implementation; JCS number formatting is the risky part to hand-roll.