Fix (#2095): Stabilize ErgoTransactionSpec by using proper Try pattern matching

[dimension-drifter]

Problem

Fixes #2095

The ErgoTransactionSpec test was unstable and occasionally failing in CI. The root cause was the use of unsafe .toEither.left.get when checking for expected exceptions in the "context extension with neg id" and "context extension with neg and pos ids" test cases.

###This pattern has a critical flaw:

• If (txTry) is unexpectedly a Success instead of Failure, calling .left.get on a Right throws NoSuchElementException
• This causes the test to fail with a confusing error message rather than the actual assertion failure
• The behavior is non-deterministic if there are race conditions or timing issues in the underlying prover logic

###Solution
Replace the unsafe .toEither.left.get pattern with proper scala.util