ODSC-85044: Improve AQUA shape recommender fallback for nested text_config model configs

[Aryanag2]

Jira

• Ticket: ODSC-85044

What Changed

• Added a nested decoder fallback in LLMConfig.from_raw_config() so recommender parsing can use text_config or similar nested text sections when top-level decoder fields are missing.
• Kept top-level config values authoritative while validating nested decoder sections so unsupported nested decoders still return descriptive AquaRecommendationError messages.
• Updated focused AQUA recommender unit coverage for supported nested text_config parsing and unsupported nested decoder validation.

Validation

• python3 -m pytest tests/unitary/with_extras/aqua/test_recommend.py -k text_config
• python3 -m pytest tests/unitary/with_extras/aqua/test_recommend.py -k valid

Risks / Follow-Ups

• The fallback only fills missing decoder fields; broader nested config handling remains unchanged.

Checklist

• PR title follows <JIRA-KEY>: <summary>
• Changes are scoped to the ticket
• Relevant tests were added or updated
• Validation steps are documented above
• Commits are signed off