Reject extra documents in parameter files

[apstndb]

Summary

• Require JSON parameter files to contain one top-level mapping and reject trailing content, a second value, and JSON null.
• Reject additional value-bearing YAML documents instead of silently ignoring parameters after the first document.
• Preserve existing empty-file, comments-only YAML, empty-document, and empty-mapping behavior with focused format-specific regression tests.

Validation

• go test ./params/...
• go test -race ./params/...
• go test -count=1 ./...
• go vet ./...
• golangci-lint run ./...
• git diff --check

[gemini-code-assist]

Code Review

This pull request updates the parameter file loading logic in params/load.go to enforce that JSON and YAML files contain exactly one top-level mapping. It adds validation to reject top-level null values and multiple documents/values. Additionally, new unit tests are introduced in params/load_test.go to verify these validation rules. There are no review comments, so I have no feedback to provide.

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