Use ConsensusFactory when parsing block headers - #1335

Summary

While reviewing the code to gain familiarity with NBitcoin I noticed what appear to be some oversights in the use of ConsensusFactory.

ConsensusFactory was introduced in 6cb308a but wasn't applied everywhere it could be. Some missed uses were corrected in 5f7c433c.

I found 3 additional missed cases in header parsing:
- BlockHeader.Parse selected a factory but directly constructed a base BlockHeader.
- raw RPC header parsing created the correct header type but did not preserve the network factory during deserialization.
- REST header parsing directly constructed base headers and split the response into fixed 80-byte chunks.

Changes

- Create statically parsed headers through ConsensusFactory.CreateBlockHeader() and deserialize them with the same factory.
- Preserve the client's network factory while parsing raw RPC headers.
- Parse REST header responses sequentially using factory-created headers, allowing custom header implementations to determine their serialized length.
- Reject truncated REST header data and guard against custom deserializers that consume no input.

No public API signatures are changed, and standard Bitcoin header serialization and parsing remain unchanged.

Tests

Added regression coverage using an artificial test-only ConsensusFactory, custom BlockHeader, and custom Block. The custom header includes an extended serialized field and a nested factory-created value, which verifies both concrete type creation and propagation of the factory into deserialization.

Post-rebase test results:

- focused ConsensusFactoryTests: 10 passed, 0 failed
- complete NBitcoin.Tests Release/net10.0 suite: 752 passed, 0 failed