fix: set errors on all commands when context is cancelled in Pipeline.Exec

[pistakugli]

Problem

When Pipeline.Exec(ctx) is called with a pre-cancelled context, the commands in the queue don't have their errors set. This causes:

• Command accumulation on pipeline reuse
• Memory leaks from unreferenced commands
• Connection pollution from unclean state

Solution

Added a context check before execution. If the context is already cancelled, we set the context error on all commands and return immediately.

Changes

• pipeline.go: Added context check in Exec() that sets errors on all commands when context is pre-cancelled
• pipeline_test.go: Added 3 tests for context cancellation scenarios

Tests

1. Context cancellation: Verifies all commands get context.Canceled error
2. Pipeline reuse: Verifies pipeline works correctly after failed Exec
3. Deadline exceeded: Verifies context.DeadlineExceeded is handled

Acceptance Criteria

• Pipeline.Exec(ctx) guarantees that the internal command queue is cleared on all exit paths
• All commands queued during a cancelled Exec have their internal error set to the context error
• Pipeline reuse after a failed/cancelled Exec starts with a clean slate

Wallet: 0xb7419c92b1d93251c0c85b756b390b29ab8162e0 (Polygon)