FIX Validate LeetspeakConverter custom_substitutions and match keys case-insensitively

[Shivam Sharma (ShivamSharma43)]

Description

Fixes #3293.

LeetspeakConverter.convert_word_async looks each character up as char.lower(), but custom_substitutions was used without validation. An uppercase key such as {"A": ["4"]} never matched, a multi-character key such as {"th": ["7"]} could never match, and an empty replacement list such as {"e": []} was accepted and then raised IndexError on the first matching character.

The converter now validates custom substitutions when it is created:

• Keys are matched case-insensitively. They are stored by lowercase character, so {"A": ["4"]} replaces both A and a, the same way the default substitutions already apply to both cases.
• A key that is not a single character, two keys that differ only in case (such as "a" and "A"), or a value that is not a non-empty list of strings raises ValueError.

Custom substitutions with lowercase single-character keys keep their output and their identifier: the custom_substitutions_hash is computed from the same mapping as before, so existing hashes do not change. {"A": ["4"]} now gets the same identifier as {"a": ["4"]}, because the two convert identically. An empty custom_substitutions dict still falls back to the defaults.

Tests and Documentation

• New tests in test_leetspeak_converter.py:
  • Uppercase keys apply in deterministic and random modes.
  • {"A": ["4"]} and {"a": ["4"]} share an identifier.
  • Invalid mappings raise ValueError at construction, in both modes: an empty list, a string value, a non-string replacement, a multi-character key, an empty key, and keys differing only in case.
  • On main, all 15 new tests fail. With this change, all of them pass.
• pytest tests/unit/converter/test_leetspeak_converter.py: 24 passed. pytest tests/unit/converter tests/unit/registry tests/unit/backend/test_converter_service.py: 3735 passed, 38 skipped.
• ruff check, ruff format and ty check pass on the changed files. git diff --check is clean.
• No documentation changes. JupyText not run, since the converter docs only use the default substitutions.

🤖 Generated with Claude Code

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