fix: reject unknown structured collection keys

[asadijabar]

Summary

• reject unknown nested keys inside direct structured map values in strict mode
• reject unknown nested keys inside structured slice elements in strict mode
• add regression coverage for both strict-mode bypasses

Validation

• go test ./... -run TestLoad_NestedCollections -count=1
• make ci (format, vet, and tests passed; lint failed because local golangci-lint was built with Go 1.25 while the active Go toolchain includes Go 1.26 files requiring a newer linter build)

[Copilot]

Pull request overview

This PR tightens strict-mode validation to reject unknown keys that appear inside directly provided structured collection values (e.g., map[string]Struct values provided as nested maps, and []Struct elements provided as nested objects), and adds regression tests to prevent strict-mode bypasses for these cases.

Changes:

• Extend strict-mode unknown-key detection to recursively validate nested keys inside structured struct, []struct, and map[string]struct values provided directly in source data.
• Add helper functions to collect valid key→type mappings and to recursively validate nested structured values.
• Add regression tests covering unknown nested keys in direct map values and slice elements under strict mode.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
loader.go	Adds strict-mode recursion to detect unknown nested keys inside structured collection values.
loader_test.go	Adds regression tests ensuring strict mode rejects unknown nested keys in direct map values and slice elements.

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[Azhovan]

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[chatgpt-codex-connector]

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Reviewed commit: 0fafc07413

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