[fix][model] read default parameter values from config

[oleg-ai-dev]

What type of PR is this?

fix

Check the PR title

• This PR title matches the format: [<type>][<scope>] <description>.
• The description of this PR title is user-oriented and clear enough for others to understand.
• No usage-level documentation is required for this configuration-decoding fix.
• This PR is written in English.

(Optional) More detailed description for this PR

en:

The shipped Docker Compose and Helm model_config.yaml files define parameter defaults with the default_val key. ParamSchema originally decoded only default_value, so Viper left every shipped default empty and the model-management response did not expose those defaults.

This change decodes default_val as the canonical shipped spelling while preserving default_value for backward compatibility with existing custom configurations. The alias is normalized from the same Viper snapshot, so live reloads cannot return mixed configuration versions. When both keys are present, default_val takes precedence; the public JSON field remains default_value.

A regression test loads real YAML through the production Viper factory and verifies both spellings, nested parameter schemas, and deterministic precedence. A second test verifies normalization uses a single loader snapshot and removes the internal alias before returning the model.

Validation

• go test -count=1 ./modules/llm/infra/config
• go test -count=1 ./modules/llm/...
• go vet ./modules/llm/...
• go build ./modules/llm/...
• git diff --check

(Optional) Which issue(s) this PR fixes

Fixes #125

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