[CuTe DSL] Reject incomplete dynamic f-string formats

[XFDG]

Summary

• reject dynamic f-string format specs that do not end in a supported printf conversion character
• prevent width-only specs such as {value:8} from producing malformed %8 device format strings that leave the argument unconsumed
• preserve valid explicit specs such as .2f and add focused regression coverage

Testing

• B200 node, Python 3.12: an AST-extracted execution of the current-main FormattedValue produced valid=%.2f width_only=%8 and exited 2
• B200 node, Python 3.12: the fixed source produced valid=%.2f width_only=rejected and exited 0
• python3 -m py_compile python/CuTeDSL/cutlass/base_dsl/ast_helpers.py test/python/CuTeDSL/test_formatted_value.py
• the source checkout was not imported as a complete DSL package because its generated cutlass._mlir extension is not built on the validation node

Contribution disclosure

AI assistance was used for repository research, implementation, and running the checks above.

Fixes #3500