fix: validate model exists in settings.toml

[NishchayMahor]

Summary

Closes #27.

Calling wraval with a --model value that has no matching [<model>] section in config/settings.toml blows up later in get_settings() at settings.endpoint_type with AttributeError: 'Settings' object has no attribute 'ENDPOINT_TYPE', because Dynaconf silently loads nothing for the missing env and the default values never get attached.

This adds a small preflight check that validates model against the section names actually present in settings.toml before Dynaconf is instantiated, and raises a ValueError listing the available models so the user can see what they typed wrong.

What changed

• src/wraval/main.py: extracted the settings file path to a module constant, added _known_models() (regex over settings.toml for section headers, ignoring [default]), and added a case-insensitive validation step inside get_settings() that matches the Dynaconf case_sensitive=False semantics introduced in Add case_insensitive=False to Dynaconf instantiation #28.
• No new dependencies. Stdlib re only.

Behavior

Before:

$ wraval inference --model totally-bogus
... (later)
AttributeError: 'Settings' object has no attribute 'ENDPOINT_TYPE'

After:

$ wraval inference --model totally-bogus
ValueError: Unknown model 'totally-bogus'. Add a [totally-bogus] section to config/settings.toml,
or pick one of: haiku-3, haiku-3-5, haiku-3-5-cross, haiku-4-5, nova-lite, phi-3-5-4B,
phi-3-ollama, phi-4-4B, qwen-2-5-1-5B, qwen-3-0-6B, qwen-3-1-7B, qwen-3-1-7B-async,
qwen-3-4B, qwen-3-4B-async, sonnet-3, sonnet-3-5.

Test plan

Repo currently has no test suite. Verified manually that:

• known model names load through (e.g. haiku-3)
• default is accepted explicitly
• case-insensitive matches go through (e.g. Haiku-3, HAIKU-3)
• unknown names raise ValueError with the full list of available sections
• empty string raises ValueError