Fix codegen imports for public tagging utilities

[Linxiushen]

Generating configuration code for fiddle.tagging.list_tags or materialize_tags currently emits nonexistent fdl attributes, so executing the fixture raises AttributeError. Select public import aliases for these two symbols while preserving existing root-package aliases and user-registered overrides.

Adds executable import-identity checks (including namespace collisions), modern configuration roundtrip/build coverage, and a legacy Partial roundtrip. Addresses the tagging-utility portion of #564; this does not close the separate TaggedValue import cases.

Validation: Python 3.12 on Linux with the full [testing] dependencies: pytest fiddle — 1,114 passed, 4 skipped, 350 subtests passed. Pytype passes for both changed production files. New regressions fail on the unchanged source; public new_codegen and auto_config_codegen generated modules also reconstruct and build successfully.

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