Reapply YAML deep-merge fix on v5

[Siddharth2207]

Summary

• Reapplies PR Fix shallow merge overwriting same-section entries in multi-document YAML #2430 on top of the v5 revert (PR Revert entire v5 branch to a044f50f #2517)
• Fixes shallow merge overwriting same-section entries when merging multiple YAML documents (e.g., two different deployers under deployers: — second one would silently clobber the first)
• Implements deep recursive merging in crates/settings/src/yaml/emitter.rs so entries under the same top-level section are properly combined

Changes

• crates/settings/src/yaml/emitter.rs: adds deep_merge_hash helper and updates emit_documents to use it; adds regression test

Test plan

• Existing emitter tests pass
• New test_emit_deep_merges_hash_sections test passes
• CI passes

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