Add module examples and submodules tools

[ayhammouda]

Summary

• add get_module_examples and get_module_submodules registry tools
• keep get_module_details focused on root module details plus example/submodule indexes
• add unit and Docker e2e coverage for listing and selecting module examples/submodules

Closes #307

Validation

• go test ./pkg/tools/registry ./pkg/tools ./pkg/toolsets
• go vet ./pkg/tools/registry ./pkg/tools ./pkg/toolsets
• go test ./cmd/terraform-mcp-server ./pkg/client ./pkg/resources ./pkg/tools ./pkg/tools/registry ./pkg/tools/tfe ./pkg/toolsets ./pkg/utils ./version
• go vet ./cmd/terraform-mcp-server ./pkg/client ./pkg/resources ./pkg/tools ./pkg/tools/registry ./pkg/tools/tfe ./pkg/toolsets ./pkg/utils ./version
• make test-e2e

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[ayhammouda]

Hi! Quick follow-up on this PR since the checks are passing now.
Happy to adjust anything if needed. Thanks!