test: add config CRUD unit tests

[MD-Mushfiqur123]

Closes #391

Adds table-driven Go tests for LocalStorage config CRUD operations: SetConfig, GetConfig, ListConfigs, DeleteConfig.

10 test cases covering:

• first insert creates version 1
• upsert increments version, preserves created fields
• list returns lexical order
• get missing key returns (nil, nil)
• delete removes entry
• delete missing key returns error
• cancelled context fails fast
• delete nonexistent config twice
• list configs when empty
• multiple upsert round-trip

cd control-plane && go test ./internal/storage/... -run Config -count=1
ok  github.com/Agent-Field/agentfield/control-plane/internal/storage

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[AbirAbbas]

@MD-Mushfiqur123 thank for the contribution, if you could sign the CLA agreement I can get started on the review