feat: allow reassigning a gateway's team (and its federated entities) from the edit form

[hirapHoriuchi]

✨ Feature / Enhancement PR

Draft — opened for feedback on the design question below before finalizing, since this touches team scoping (RBAC visibility).

🔗 Epic / Issue

Closes #5792

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🚀 Summary (1-2 sentences)

Adds a team selector to the MCP server (gateway) edit form and makes team reassignment consistent by propagating the new team to the gateway's federated tools/resources/prompts. Mirrors the virtual server selector in #5791, plus the backend propagation gateways need.

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❓ Design question for maintainers (please confirm before merge)

When a gateway's team changes, this PR propagates the new team_id to child tools/resources/prompts that inherited the old team, preserving per-item overrides (mirroring how visibility already propagates). Is that the desired behavior, or should children be left in place and moved individually? Happy to change the approach based on your preference — that's why this is a draft.

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📏 Reviewability

• This PR has one clear purpose
• The linked issue is not labeled triage
• Unrelated bugs or improvements are tracked in separate issues/PRs
• Tests are included with the code they validate

Notes:

• The UI-only virtual server counterpart is a separate PR ([FEATURE]: Allow selecting the team when editing a virtual server #5791). This PR is the gateway case, which additionally needs backend changes.
• The linked issue was just opened and is not yet maintainer-labeled; opening as a draft to provide concrete code and drive the design discussion.

What it does

UI (mirrors the virtual server selector): owned-teams-only dropdown, empty placeholder to avoid implicit moves, team_id sent only for team visibility, submission blocked with an accessible message otherwise, disabled with an explanation when the user owns no teams.

Backend (update_gateway):

• Validates destination-team ownership before the duplicate checks.
• Runs the slug/URL duplicate checks against the destination team when the team is changing (previously used the pre-change team).
• On team change, propagates the new team_id to child tools/resources/prompts that inherited the old team, preserving per-item overrides. No schema/migration change (columns already exist).

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🧪 Checks

Targeted checks run locally (not the full make lint/make test suite):

• python -m pytest tests/unit/mcpgateway/services/test_gateway_service.py → 354 passed, 1 skipped (includes new deny-path, propagation-with-override, and destination-team conflict tests)

• npx vitest run tests/unit/js/gateway.test.js tests/unit/js/formSubmitHandlers.test.js → 116 passed

• ruff check mcpgateway/services/gateway_service.py → clean

• make build-ui → success

• Manually verified end-to-end: reassigning a gateway's team on a local instance moved the gateway and its two federated tools to the destination team together; per-item overrides were preserved.

• make lint passes (ran targeted ruff/eslint on changed files only)

• make test passes (ran targeted pytest/vitest above)

• CHANGELOG updated (if user-facing) (can add an entry if preferred)

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📓 Notes (optional)

Security: destination-team ownership is validated before applying the change and cannot be bypassed via disabled gateways (validation and assignment are under the same enabled/include-inactive guard). Deny-path regression tests are included.