generators: add option to skip output btw given delimiters; add hook for postprocessing

[leondz]

1. add generators.base.Generator._prune_skip_sequences() to cut content btw Generator.skip_seq_start and Generator.skip_seq_end
2. add generators.base.Generator._post_generate_hook() to allow postprocessing of output in generators
3. factor "test" generators out into their own module, for consistency w other plugin types
4. handle openai exceptions caused by interrupted thought (e.g. when hitting max_tokens)
5. add configurability to OpenAICompatible

Verification

• python -m pytest tests/generators/test_generators_base.py
• python -m pytest tests/generators/test_generators.py
• Verify Run something with test.Repeat, it should work as normal (output==prompt)
• Verify Configure skip_seq_start and skip_seq_end, content int he middle should be pruned
• Docs updated in docs/source/garak.generators.base.rst

[jmartin-tech]

Testing shows this works however the long length of responses often truncates before the </think> output, often leaving the full response that started with <think>... in the result.

Also noted that I do not believe timeout should be suppressed by default for OpenAICompatible.