[Java][BigQueryIO] Honor allowMissingRequiredFields when building Storage Write API messages

[khaledh]

fixes #40189

When allowMissingRequiredFields is true, conversion already skips the missing-required check, but both build sites still call build(), which re-validates the whole tree and can throw. This makes the flag consistent at build time:

return allowMissingRequiredFields ? builder.buildPartial() : builder.build();

• false path unchanged; a nested test pins the exact SchemaMissingRequiredFieldException diagnostic.
• In-tree callers passing true already use all-optional descriptors: identical output, one redundant isInitialized() walk removed.
• Behavior change, declared in CHANGES.md: an external caller passing true with a strict descriptor now gets a partial message instead of UninitializedMessageException. A one-time descriptor check can be added if preferred.
• Dataflow, production workload: −12% CPU/record alone; −34% CPU and +46% throughput combined with [Feature Request]: Avoid per-field Descriptor.findFieldByName lookups in BigQuery Storage Write API TableRow conversion #40188.

Tests: on/off parity for all-optional descriptors including unknown fields, strict-path failure unchanged, nested diagnostic chain, partial messages for map and positional rows. GCP IO suite green on JDK 11 and 17.

Related: #40188 (independent; either order).

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[kennknowles]

R: @ahmedabu98

Sorry to send so many to you, but am I right that you are the current expert in the storage write API paths?

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