fix(spanner): compare null elements in primitive-array Value.equals

[fornwall]

The five primitive-array Value implementations (BoolArrayImpl, Int64ArrayImpl, Float32ArrayImpl, Float64ArrayImpl, PgOidArrayImpl) implemented valueEquals as Arrays.equals(values, that.values) on the backing primitive array only.

Null elements are tracked in a separate BitSet nulls and stored as the primitive default (0, 0.0, false) in the backing array, so the null mask was never part of equality:

 // Returned true:
Value.int64Array(Arrays.asList(null, 1L)).equals(Value.int64Array(Arrays.asList(0L, 1L)))

Consequences:

• equals returned the wrong answer — it reported semantically distinct values as equal: [null, 1] and [0, 1] serialize to different protos via valueToProto() (null elements become NULL, not the primitive default) and are visibly different through getInt64Array().
• equals/hashCode contract violation — PrimitiveArrayImpl.valueHash() does include the null mask (31 * Objects.hashCode(nulls) + arrayHash()), so values that compared equal hashed differently, breaking HashMap/HashSet usage of Value, Struct, and Mutation.
• Wrong mutation diffs — the Cloud Spanner Import/Export Dataflow template relies on Mutation.equals to detect modifications before committing. An update that changes [0, 1] to [NULL, 1] (or vice versa) compared as "no change" and can be silently dropped.

Fix that by moving the null-mask comparison into a single shared valueEquals override on PrimitiveArrayImpl, mirroring the existing valueHash()/arrayHash() split, and turn the per-subclass valueEquals overrides into arrayEquals.

[gemini-code-assist]

Code Review

This pull request fixes an issue in Value.java where equality checks for primitive arrays did not correctly account for null elements. Since null elements are stored as primitive defaults in the backing array, the valueEquals method in PrimitiveArrayImpl was updated to compare both the nulls bitmask and the backing arrays via a new abstract arrayEquals method. Corresponding subclass implementations were updated, and comprehensive equality tests were added in ValueTest.java to verify correct behavior. There are no review comments, so I have no feedback to provide.