[architect] Extract core resource handlers from pkg/api/handlers/mcp_resources.go

[clubanderson]

Part of #19063

Scope

Move the core-resource read handlers out of pkg/api/handlers/mcp_resources.go into a focused file such as pkg/api/handlers/mcp_resources_core.go.

Target files

• pkg/api/handlers/mcp_resources.go
• pkg/api/handlers/mcp_resources_core.go

Functions in scope

• GetConfigMaps
• GetSecrets
• GetServiceAccounts

Acceptance criteria

• Core-resource handlers no longer live in the catch-all file
• Any tiny helpers used only by those handlers move with them
• Behavior and existing tests stay unchanged

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