test: add test for withdraw() at same timestamp as resume()

[GazzyLee]

Summary

Add a dedicated withdraw_at_same_timestamp_as_resume() test to tests/stream_pause_resume.rs.

This test covers calling withdraw() at the exact same ledger timestamp as resume(), with zero elapsed seconds since the stream was resumed.

It verifies that the withdrawable amount correctly reflects zero newly streamed tokens at that instant.

Motivation

None of the existing tests in stream_pause_resume.rs covered the edge case of calling withdraw() at the exact same timestamp as resume().

This dedicated test guards against potential off-by-one errors in the elapsed-time calculation by ensuring that when a withdrawal occurs immediately after resuming, with no time elapsed, only the tokens accumulated before the pause are withdrawable.

Test Details

The new test:

• Creates a stream and allows it to run for 300 seconds, resulting in 300,000 tokens streamed.
• Pauses the stream for 1,000 seconds. This paused duration should not contribute to the streamed amount.
• Resumes the stream.
• Immediately calls withdraw() at the exact same ledger timestamp as resume(), without calling advance().
• Asserts that only the 300,000 tokens accumulated before the pause are withdrawable.
• Verifies that zero newly streamed tokens are included at that instant.

Test Results

All 8 tests in stream_pause_resume.rs pass, including the new test:

running 8 tests
test withdraw_at_same_timestamp_as_resume ... ok
test resume_on_running_stream_is_rejected ... ok
test pause_freezes_withdrawable_amount ... ok
test double_pause_is_rejected ... ok
test paused_time_excluded_from_streamed_total ... ok
test multiple_pause_resume_cycles_accumulate_correctly ... ok
test resume_after_pause_continues_stream_correctly ... ok
test recipient_can_withdraw_accumulated_balance_while_paused ... ok

test result: ok. 8 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out


## Closes #228 

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