Closes #2563

This PR primarily fixes Issue #2563, where Gateway tags were returned as an empty list despite being present in the database.
During unit testing for this fix, an additional but related issue was discovered in the Gateway update flow, which is also addressed here.

Primary Issue (#2563): Gateway Tags Returned as Empty List

Problem

Gateway APIs (GET / PUT / PATCH) returned tags: [] even when valid tag data existed in the PostgreSQL JSON column.

Root Causes

1. Type annotation mismatch
   validate_tags_field() was annotated as returning List[str] but actually returned List[Dict[str, str]], causing Pydantic serialization/coercion issues.

2. Dict input destruction during validation
   TagValidator.validate_list() stringified dict inputs (e.g. {"id": "finance", "label": "Finance"}), which then failed validation and were filtered out.

3. Missing legacy tag normalization
   _prepare_gateway_for_read() did not convert legacy string tags to dict format before validation.

Fixes Implemented

mcpgateway/validation/tags.py

• Corrected validate_tags_field() return type to List[Dict[str, str]]

• Added passthrough logic for already-formatted tag dictionaries:

  if isinstance(tags[0], dict):
      return [t for t in tags if isinstance(t, dict) and "id" in t and "label" in t]

mcpgateway/schemas.py

• Updated GatewayCreate.tags and GatewayUpdate.tags to accept both legacy and new formats:

  tags: Optional[List[Union[str, Dict[str, str]]]] = Field(...)

• Aligned validator return type annotations with actual return values

mcpgateway/services/gateway_service.py

• Enhanced _prepare_gateway_for_read() to normalize legacy string tags into dict format prior to validation

Result

Gateway tags now serialize and deserialize correctly across all API endpoints, preserving both id and label fields from the database.

Additional Issue Found During Unit Testing: Transport Field Reset

Problem

While validating the tags fix via unit tests, it was observed that updating a gateway without explicitly providing transport caused the field to reset to "SSE" instead of preserving the existing value (e.g. "STREAMABLEHTTP").

This behavior occurred only via the API; the Admin UI was unaffected.

Root Cause

In GatewayUpdate schema:

transport: str = "SSE"

This caused omitted fields in PUT / PATCH requests to overwrite existing values, which violates REST API semantics.

Fix Implemented

• Updated GatewayUpdate.transport to default to None:

  transport: Optional[str] = Field(None, ...)

• Ensures existing values are preserved unless explicitly provided in the request

Testing

Verified via curl:

# Before fix:
# - transport reset to SSE
curl -X PUT \
  'http://localhost:4444/gateways/{id}' \
  -H 'Content-Type: application/json' \
  -H "Authorization: Bearer $TOKEN" \
  -d '{"name":"updated-name"}'

After the fix:

• Tags are preserved and correctly returned
• transport remains unchanged unless explicitly passed