service: avoid shared label-map mutation in worker gateway generation

[ShubhamMishra99]

Description

This PR fixes a reconciliation-safety issue in worker gateway topology generation caused by mutation of caller-owned label maps during gateway object construction.

buildMinimumGateway previously copied the provided labels into a local gatewayLabels map, but also mutated the original input map by injecting controller/project/slice labels directly into labels.

Because the same label map can be reused across full-mesh gateway pair generation during SliceConfig reconciliation, this introduced hidden shared mutable state and weakened deterministic topology object synthesis.

Changes

• Stop mutating the caller-provided labels map in buildMinimumGateway

• Apply controller/project/slice labels only to the local gatewayLabels copy

• Extend regression assertions validating that:

  • owner labels remain unchanged
  • no reconciliation-specific labels leak into caller-owned maps
  • original label map size/content remains intact

Why this matters

This improves:

• reconciliation determinism
• helper purity/idempotency
• topology generation safety
• maintainability of gateway object synthesis logic

The change also reduces hidden coupling between topology intent inputs and generated gateway objects during full-mesh reconciliation workflows.

Testing

Updated:

• testBuildMinimumGatewayLabelsAreIsolatedPerGateway

Additional assertions now verify:

• no mutation of caller-owned labels
• no leaked reconciliation labels
• original label content remains unchanged after generating multiple gateway objects

Compatibility

• No API changes
• No CRD changes
• No intended topology behavior changes
• Only removes unintended side effects during gateway construction

Architectural Context

SliceConfig reconciliation computes desired topology intent and generates full-mesh worker gateway objects across clusters. Helper-level mutation of shared label state breaks clean intent-to-object boundaries and can introduce inconsistent reconciliation behavior over time.

This fix restores safer object materialization semantics by ensuring gateway construction operates on isolated label copies.