for string registration, move arguments from a dictionary to kwargs and convert sugar syntax to underscores

Greptile Summary

This PR refactors the gt-i18n string registration interface by renaming all $-prefixed GT reserved keys to underscore-prefixed equivalents ($context → _context, $id → _id, $max_chars → _max_chars, $_source → __source, $_hash → __hash, $_fallback → __fallback), making kwargs valid Python identifiers. It also addresses previous review feedback by replacing the brittle prefix-based variable filtering with an explicit _GT_RESERVED_KEYS blocklist, and by extracting duplicated test helper code into a shared helpers.py module.

Key changes:

• _extract_variables.py: New _GT_RESERVED_KEYS frozenset replaces startswith("$") filter — user variables with _ prefixes are no longer silently dropped
• _msg.py, _t.py, _interpolate.py, _is_encoded.py: All internal key lookups updated to new naming convention
• tests/helpers.py + tests/conftest.py: Shared test utilities extracted (resolving the duplicated-helper concern from the prior review)
• Minor docstring inconsistency in _interpolate.py: step 1 still describes the old prefix-filtering behavior rather than the new explicit blocklist

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["t(message, **kwargs)\n_context, _id, _max_chars"] --> B["hash_message()\nuses _context, _id, _max_chars"]
    A --> C{"translation found?"}
    C -->|yes| D["interpolate_message(translated,\n{**kwargs, __fallback: message})"]
    C -->|no| E["interpolate_message(message, kwargs)"]
    D --> F["extract_variables(options)\nblocks _GT_RESERVED_KEYS"]
    E --> F
    F --> G["user vars only\n(e.g. name, count, _custom_var ✓)"]
    F --> H["GT reserved keys removed\n_context, _id, _max_chars\n__hash, __source, __fallback"]
    D --> I{"format error?"}
    I -->|"__fallback present"| J["retry with source msg\n(__fallback=None)"]
    I -->|no fallback| K["return raw + _max_chars cutoff"]
    J --> L["interpolated result"]
    G --> L

_{Last reviewed commit: 1da7587}

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