fix(plugin-multi-tenant): serialize transaction dataloader finds

[cyphercodes]

What?

Serialize payloadDataLoader.find calls only while a request is running inside a transaction. This prevents multiple relationship/filterOptions validators from issuing concurrent MongoDB operations on the same transaction session.

Why?

The multi-tenant plugin auto-injects relationship filterOptions. Array rows are validated in parallel, so creating a tenant-scoped document with several relationship rows can trigger concurrent payload.find calls in the same MongoDB transaction and surface false invalid relationship validation errors.

How?

• Add a per-request find queue in getDataLoader for transactional finds.
• Leave non-transactional finds concurrent.
• Add unit coverage for transactional serialization and non-transactional concurrency.
• Add a multi-tenant integration regression covering array relationship creation with same-tenant items.

Fixes #16462

Verification

• pnpm exec vitest run --project unit packages/payload/src/collections/dataloader.spec.ts
• pnpm exec vitest run --project int test/plugin-multi-tenant/int.spec.ts -t 'should create an array of same-tenant relationships without transaction races' --no-cache
• pnpm exec vitest run --project int test/plugin-multi-tenant/int.spec.ts --no-cache
• pnpm exec prettier --check packages/payload/src/collections/dataloader.ts packages/payload/src/collections/dataloader.spec.ts test/plugin-multi-tenant/int.spec.ts
• pnpm exec eslint --flag v10_config_lookup_from_file packages/payload/src/collections/dataloader.ts
• pnpm exec eslint --flag v10_config_lookup_from_file --no-warn-ignored packages/payload/src/collections/dataloader.spec.ts
• NODE_OPTIONS=--max-old-space-size=4096 pnpm exec eslint --flag v10_config_lookup_from_file --cache --cache-strategy content --no-error-on-unmatched-pattern test/plugin-multi-tenant/int.spec.ts (passes with existing warnings for token and blueDogTenantID)
• pnpm --filter payload build:swc
• git diff --check

Note: a combined ESLint run over both changed non-spec files hit local Node heap/timeout limits, so I verified the files separately.

[amitzur]

@cyphercodes thanks for the quick turnaround!