fix(server): let load args override saved args

[fl0rianr]

Summary

Make explicitly provided *_args in /load replace the saved value for the same key before merge_args is applied. Lower-priority architecture, backend, and global args can still be merged normally.

Follow-up/related to #3082.

Scope

• This PR addresses one clear issue or change.
• I reviewed the full diff myself before submitting.
• I removed unrelated local changes.
• I kept refactoring separate unless it is required for this change.

Testing

• Code builds without errors locally.
• I described the testing performed below.

Testing details:

• Updated test_012lc to verify request llamacpp_args replace the saved same-key layer without modifying the saved value.
• Added test_012le to verify request args still merge with lower-priority global args and win on conflicting flags.

Documentation

• Documentation is not affected by this change.

Breaking Changes

• This PR does not introduce breaking changes.

AI-assisted contribution

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If AI tools were used:

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• I checked for hallucinated APIs, unrelated changes, and incorrect assumptions.

[bitgamma]

LGTM