fix(plugins): allow complete LoggingPlugin output

[Manitchahar]

Link to Issue or Description of Change

• Closes: What's the purpose of logging if messages are truncated ? At least it should be configurable #6056
• Replaces fix(plugins): allow complete LoggingPlugin output #7079, which was closed automatically when its source fork was deleted. Same change, rebased onto current main.

Problem:
LoggingPlugin hard-codes truncation at 200 characters for text parts and system instructions, and 300 for tool arguments/results. Error details at the end of a long tool result never reach the log.

Solution:
Add two keyword-only constructor options. Defaults are unchanged; None disables truncation.

LoggingPlugin(max_content_length=None, max_args_length=None)

Only logging_plugin.py and its test file are touched.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

tests/unittests/plugins/test_logging_plugin.py: 31 passed on current main (20 new cases covering custom limits, exact-boundary lengths, zero, and None). Previously verified on Python 3.10–3.14.

Manual End-to-End (E2E) Tests:

Ran a real InMemoryRunner with a scripted local BaseLlm that calls a tool returning a long diagnostic. A marker placed after the default cutoff is hidden with the defaults and visible with custom or None limits, for tool results, model responses and system instructions.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective.
• New and existing unit tests pass locally with my changes.

Additional context

Companion docs PR: google/adk-docs#2215

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