refactor: remove obsolete dynamic checkpoint mismatch handling (cleanup PR #4099)

[RexBearIU]

Description

This PR cleans up the dynamic checkpoint mismatch error-handling logic originally introduced in PR #4099. Since the proactive validation of scan_layers from checkpoint metadata (PR #4304) catches mismatches early, the after-the-fact recovery logic is no longer needed.

Key Changes:

• Removed obsolete helpers (handle_checkpoint_mismatch, is_structural_or_shape_mismatch) from checkpointing.py.
• Simplified the from_pretrained loading block in model_creation_utils.py.
• Deleted obsolete unit tests (TestCheckpointMismatchHandling and related test cases) in checkpointing_nnx_load_test.py.
• Updated test_scan_layers_mismatch_tpu in checkpointing_test.py to assert on proactive verification error messages.
• Fixed an f-string quoting compatibility issue in inspect_checkpoint.py to allow pylint to pass on Python < 3.12.

Dependency Note:

⚠️ This PR must be merged AFTER PR #4304 (which implements the proactive metadata-based validation).

Tests

Tested this change by running the following unit and integration tests under the CPU platform:

• tests/unit/checkpointing_nnx_load_test.py
• tests/unit/model_creation_utils_test.py
• tests/post_training/unit/lora_utils_test.py
• tests/integration/checkpointing_test.py

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

❌ Patch coverage is 72.17391% with 32 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/utils/model_creation_utils.py	78.94%	14 Missing and 6 partials ⚠️
src/maxtext/common/checkpointing.py	42.10%	10 Missing and 1 partial ⚠️
...axtext/checkpoint_conversion/inspect_checkpoint.py	0.00%	1 Missing ⚠️

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