[ANCHOR-1296]: SEP-6 wallet PATCH, quote funding-method conflict & claimable-balance non-support

[ceciliaromao]

Description

• Adds PATCH /sep6/transactions/{id}, the wallet-facing endpoint SEP-6 defines for answering a pending_transaction_info_update request. Without it, a SEP-6 transaction the business server puts into that status has no way out — the wallet had nothing to PATCH against.
• Adds a guard in the Platform API: a PATCH /transactions call can no longer move a SEP-6 transaction into pending_transaction_info_update with an empty required_info_updates, which would have stranded it the same way (mirrors the existing SEP-31 guard).
• Adds a check that deposit-exchange/withdraw-exchange's funding_method matches the SEP-38 quote's delivery method, and rejects the request with a 400 when it doesn't.
• Adds end-to-end coverage for quote-rejection paths (Quote not found, asset/amount mismatch) that already worked but had no HTTP-level test, and for claimable-balance non-support (a wallet declaring claimable_balances_supported=true against an anchor that doesn't support it still gets a normal deposit).

Context

• Closes the last three gaps this branch owned per the SEP-6 coverage audit (test/anchor-1296-sep6-coverage, [ANCHOR-1296]: Close remaining SEP-6 coverage audit gaps #2031): the wallet PATCH, the quote funding-method conflict, and the claimable-balance non-support check.
• Feature spec, design and task breakdown: .specs/features/sep6-patch-quote-claimable/ (30 acceptance criteria, S6PQ-01..30).

Testing

• ./gradlew test
• ./gradlew spotlessCheck :core:test :platform:test
• ./gradlew :essential-tests:test --tests "*Sep6Tests*" — run twice in a row (67/67 passing)
• ./gradlew :essential-tests:test --tests "*Sep6PlatformApiTests*" --tests "*Sep38Tests*" — passing, except one pre-existing, order-dependent flake unrelated to this diff (see Known limitations)
• Independent Verifier pass (spec-anchored outcome check across all 30 acceptance criteria + a discrimination-sensor mutation test, 3/3 mutations killed) and a 6-dimension pre-push review (security / quality / architecture / performance / requirements / regression) — no findings

Documentation

• New Platform API field PlatformTransactionData.fields: the values a SEP-6 wallet supplied through PATCH /transactions/:id. Read-only — the deprecated PATCH /transactions never reads it back.
• SEP-6 transactions now also expose required_info_message/required_info_updates through the Platform API (previously mapped for SEP-31 only).
• Behavior change: the Platform API now rejects a PATCH /transactions that would put a SEP-6 transaction into pending_transaction_info_update with nothing requested.
• Behavior change: deposit-exchange/withdraw-exchange now reject a funding_method that conflicts with the referenced quote's delivery method.
• New Flyway migration V36__sep6_add_fields.sql.

Known limitations

• Two concurrent PATCHes on the same SEP-6 transaction can surface as an unhandled 500 (an OptimisticLockingFailureException from the @Version lock) instead of a mapped 400, the same exposure SEP-31's wallet PATCH already has. Not covered here — tracked as a follow-up decision, not part of this fix.
• Observed one intermittent, pre-existing failure in Sep6PlatformApiTests unrelated to this diff: shared KYC state on a hardcoded test account, reused across all PlatformApiTests subclasses on a long-lived local stack. Root-caused via a JSONCompareMode.LENIENT assertion this diff's field additions can't affect; expected to be clean on CI's fresh containers.