Bolt: make writeTo() the sole header authority in PackStreamStructure (drop getFieldCount()/getSignature())

[robfrank]

Motivation

After #5001 (PR #5059) added Bolt 5.x version-gated wire encoding, the PackStreamStructure accessors getFieldCount() and getSignature() are inconsistent with writeTo() on the version-gated structures:

• BoltNode / BoltRelationship / BoltUnboundRelationship: getFieldCount() returns the Bolt 4.x base count (3 / 5 / 3) while writeTo() emits 4 / 8 / 4 when the negotiated major is >= 5 (adding element_id fields).
• BoltDateTimeStructure: getSignature() returns the legacy 'F'/'f' signature while writeTo() emits the UTC 'I'/'i' signature on Bolt 5.0+.

This is inert today and was verified during review: PackStreamWriter serializes structures exclusively through writeTo() and never consults getFieldCount()/getSignature(); the only self-consumer of the writeStructureHeader(getSignature(), getFieldCount()) idiom is BoltPointStructure.writeTo (a version-invariant struct). The version-gated structs already document, on each accessor, that writeTo() is authoritative.

It remains a latent footgun: any future serialization path that calls writeStructureHeader(getSignature(), getFieldCount()) for a version-gated struct would emit a header/body mismatch on a 5.x connection and corrupt the stream. The accessors cannot be made version-correct because they are no-arg interface methods with no access to the negotiated version (only writeTo() has it, via the writer). Raised repeatedly by Claude and Gemini reviews on PR #5059; deferred out of that PR because it is a serialization-layer refactor, not a protocol-negotiation change.

Proposed design

Make writeTo() the sole authority for structure header emission:

1. Remove getSignature() and getFieldCount() from the PackStreamStructure interface.
2. Inline the signature + field count directly in each writeTo():
   • BoltPointStructure.writeTo (currently the only caller of the idiom) inlines its own version-invariant signature/field count.
   • The other implementers (BoltNode, BoltRelationship, BoltUnboundRelationship, BoltPath, BoltTemporalStructure, BoltDateTimeStructure) drop the two now-unused overrides; their writeTo() already writes the header directly.
3. Update BoltStructureTest assertions that currently read getFieldCount() / getSignature() to assert on the serialized wire bytes instead (the header byte + signature already asserted for the 5.x cases), or drop those assertions where redundant.

Acceptance criteria

• PackStreamStructure no longer exposes getSignature() / getFieldCount(); writeTo() is the only header contract.
• No behavioral change on the wire for any negotiated Bolt version (3.0-5.4) - guarded by the existing bolt unit + real-driver ITs.
• mvn -pl bolt verify green.

Dependencies

Follow-up to #5001 (PR #5059). Related to the Bolt certification epic #4882 / tracking issue #4890.

Effort estimate

S