fix: use Option<CryptoHash> for Transaction block_hash

[r-near]

Summary

Fixes #134 — the TryFrom<SignedTransactionView> conversion was placing the transaction hash into the block_hash field, since the RPC response doesn't include the block hash used at signing time. This caused SignedTransaction::get_hash() to return incorrect values.

• Changes block_hash from CryptoHash to Option<CryptoHash> on TransactionV0 and TransactionV1, forcing consumers to handle the missing case
• Uses custom borsh serialize_with/deserialize_with to preserve the on-chain wire format (panics with a clear message if you try to serialize a None block hash)
• Pre-populates the SignedTransaction hash cache with the correct tx hash from the RPC response
• Adds Transaction::block_hash() accessor returning Option<CryptoHash>

Breaking change: block_hash field type changed from CryptoHash to Option<CryptoHash> on TransactionV0/TransactionV1.

Test plan

• cargo check passes
• cargo test --lib passes (all 40 tests)
• Verify downstream consumers handle the new Option type

[codecov]

Codecov Report

❌ Patch coverage is 51.42857% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.90%. Comparing base (37fee2c) to head (26336cf).

Files with missing lines	Patch %	Lines
types/src/transaction/mod.rs	51.42%	17 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #135   +/-   ##
=======================================
  Coverage   50.90%   50.90%           
=======================================
  Files          40       40           
  Lines        5265     5298   +33     
=======================================
+ Hits         2680     2697   +17     
- Misses       2585     2601   +16     

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