[Python] Improve GroupByKey coder validation errors

[goutamadwant]

Summary

Improve GroupByKey coder validation errors by reporting the specific mismatched coder IDs or invalid output coder URN.

The previous validation combined three conditions into one error and passed its formatting arguments in the wrong order. This change gives each condition a focused message and adds regression coverage for valid coders, mismatched key coders, non-iterable output coders, and mismatched value coders.

Fixes #27929

Testing

• pytest -q sdks/python/apache_beam/runners/pipeline_utils_test.py (8 passed)
• pytest -q sdks/python/apache_beam/runners/portability/fn_api_runner/fn_runner_test.py::FnApiRunnerTest::test_group_by_key (1 passed)

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