Add tests for GET /block/:hash/txid/:index

[agoodminute]

New tests cover error handling for GET /block/:hash/txs/:start_index

• Returns 400 when block hash format is invalid
• Returns 404 when hash is valid but block does not exist

Why this was added:

• To lock in correct API semantics (400 for bad input, 404 for missing resource)
• To ensure missing blocks are not incorrectly returned as 400

[philippem]

Test failures are expected. Please add a fix to the underlying REST method as a second commit (then the tests will pass).