fix: return entire history when history_length=0

[lkawka]

Description

The specification states that a history length of 0 should return unlimited results (see code). However, this was recently changed to return 0 results. This fix restores the correct behavior.

Please note that there is an outstanding proposal to change this behavior. See a2aproject/A2A#1071 for more details.

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Summary of Changes

Hello @lkawka, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical bug where the history_length parameter, when set to 0, incorrectly truncated the task history to an empty list instead of returning the complete history as specified. The changes ensure that a history_length of 0 now correctly retrieves all available history, aligning the implementation with the intended behavior.

Highlights

• Bug Fix: History Length Interpretation: Corrected a bug where specifying history_length=0 incorrectly resulted in an empty history being returned, instead of the full history as per specification.
• Logic Adjustment in apply_history_length: Modified the apply_history_length utility function to explicitly check for history_length > 0 before applying history truncation, ensuring that a history_length of 0 bypasses the limiting logic and returns the complete history.
• Unit Test Updates: Updated relevant unit tests in test_default_request_handler.py to reflect the corrected behavior, changing test cases that previously used history_length=0 to history_length=1 to accurately verify positive history length limiting.

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[gemini-code-assist]

Code Review

This pull request correctly restores the specified behavior for history_length=0, ensuring that the entire task history is returned instead of an empty list. The change in src/a2a/utils/task.py is logical and directly addresses the bug. My main feedback is related to testing: the existing tests for history_length=0 were modified to test history_length=1. While this is a valid test case, it's crucial to add new, dedicated tests to verify the history_length=0 behavior to prevent future regressions of the issue this PR is fixing.