Improve context propagation consistency in transaction submission flow

## Problem

The transaction submission flow relies on context.Context for cancellation and timeout handling, but its usage is not fully consistent across all stages.

In distributed systems, proper context propagation is important to ensure that operations terminate early when cancelled and do not perform unnecessary work.

## Proposed Solution

Review and improve context handling across the submission flow, including:

* ensuring context is checked before performing expensive operations
* ensuring cancellation and timeout errors are consistently propagated
* standardizing how context-related errors are handled across different stages

## Scope

* Review SubmitTransaction, SubmitTransactionWithWait, and broadcast logic
* Improve consistency of context usage
* No changes to public APIs

## Benefits

* More predictable cancellation behavior
* Improved resource usage
* Better alignment with Go best practices for context handling