{Misc.} Fix help % token handling for Python 3.14 argparse compatibility

[YangAn-microsoft]

Summary

This PR fixes Knack help rendering for %-based help strings to keep behavior stable with Python 3.14 argparse validation.

Why this is needed (Python 3.14)

Python 3.14 argparse performs stricter help-format validation.
Help text containing unescaped literal % can raise formatting errors during help processing.
Since Knack renders help through custom paths, we must handle these cases safely while preserving argparse placeholder semantics.

What changed

1. Added safe % sanitization for argparse-facing help text.
2. Expanded argparse placeholders in Knack-rendered help output.
3. Added safe fallback behavior so malformed/literal % does not crash help rendering.
4. Added/updated tests for:
   • %(default)s
   • %(prog)s
   • %%
   • single % fallback behavior

Resulting behavior

1. %(default)s expands to the argument default value.
2. %(prog)s expands to the parser program string.
3. %% is rendered as %.
4. single % does not crash and is shown as %.

Validation

1. Targeted token tests passed.
2. Full Knack test suite passed on Python 3.14 (245 passed).
3. Azure CLI help-path checks with local Knack changes completed without parser/help crashes.

Scope

This is a focused compatibility and robustness fix for help rendering behavior under Python 3.14.