fix: retry mirrornode transaction-record 404s with exponential backoff

#353 (predicates epic) must be fully complete before starting this task.

JS reference: [PR #978](https://github.com/hashgraph/hedera-agent-kit-js/pull/978)

## Scope

- Add `_fetch_json_with_retry(url, max_retries=5, backoff_base=1.5)` helper in `hedera_mirrornode_service_default_impl.py`
- Apply retry logic to `get_transaction_record` / `get_transaction_details` only — these endpoints return 404 while the record is still propagating
- All other endpoints keep the current eager-raise behaviour

First confirm that the predicates epic in #353 is complete, then read hedera_mirrornode_service_default_impl.py and the referenced JS PR #978. Start at get_transaction_record and get_transaction_details; done means their 404 responses retry with exponential backoff, while all other endpoints retain eager-raise behavior.

Written by the indexing model from the issue text.