zebra: Move allow-external-route-update to mgmt frontend side

[donaldsharp]

The allow-external-route-update command was being compiled into the zebra side of the nb code. Thus when configuration was being applied that uses mgmtd as a frontend and zebra as the frontend one would get there first and lock the database, preventing the other side from working. Move this command to the correct spot.

[donaldsharp]

I'm moving the first commit from #21252 over to it's own commit. That particular commit fixes the mgmt_config frequent test failure we are seeing. The other PR has a bunch of new code that I would like to have more time to figure out how to get right based upon @choppsv1 and greptile's reviews.

[greptile-apps]

Greptile Summary

This PR relocates the allow-external-route-update CLI command from the zebra VTY layer (zebra_vty.c / zebra_vty_init()) to the mgmtd CLI frontend layer (zebra_cli.c / zebra_cli_init()), resolving a database lock contention when both mgmtd and zebra tried to apply the same YANG path.

• Root cause fixed: zebra_vty_init() is invoked by zebra/main.c (zebra daemon), while zebra_cli_init() is invoked by mgmtd/mgmt_vty.c (management daemon). Having the command installed in the zebra path caused it to race with the mgmtd path and lock the YANG datastore.
• Command consolidation: The two separate handlers — allow_external_route_update_cmd and no_allow_external_route_update_cmd — are replaced with a single idiomatic [no] allow-external-route-update DEFPY_YANG, which is the standard pattern used throughout zebra_cli.c.
• No functional changes: The NB callbacks (zebra_allow_external_route_update_create / zebra_allow_external_route_update_destroy) and the cli_show write callback remain unchanged.
• No dangling references: All references to the old no_allow_external_route_update_cmd symbol are gone; the test suite (test_zebra_rib.py) tests only the config string, so it remains valid.

Confidence Score: 5/5

• This PR is safe to merge — it is a targeted relocation of a CLI command with no logic changes.
• The change is minimal and surgical: one DEFPY_YANG definition moved between files, two commands collapsed into one using the standard [no] pattern, and the install site updated to match. The NB callbacks, YANG model, and test coverage are all untouched. No regressions are expected.
• No files require special attention.

Important Files Changed

Filename	Overview
zebra/zebra_cli.c	Adds a combined [no] allow-external-route-update DEFPY_YANG command and registers it in zebra_cli_init() — the mgmtd-side initializer. Correctly consolidates two separate positive/negative command handlers into one idiomatic [no] variant.
zebra/zebra_vty.c	Removes the two separate allow_external_route_update / no_allow_external_route_update DEFPY_YANG definitions and their install_element calls from zebra_vty_init(). Clean removal with no dangling references.

Sequence Diagram

sequenceDiagram
    participant Operator
    participant mgmtd
    participant zebra

    Note over mgmtd,zebra: Before fix — command registered in BOTH paths
    Operator->>mgmtd: allow-external-route-update
    mgmtd->>mgmtd: zebra_cli_init() → locks YANG DB
    Operator->>zebra: allow-external-route-update
    zebra->>zebra: zebra_vty_init() → tries to lock YANG DB ❌ deadlock

    Note over mgmtd,zebra: After fix — command registered only in mgmtd path
    Operator->>mgmtd: allow-external-route-update
    mgmtd->>mgmtd: zebra_cli_init() → locks YANG DB ✅
    mgmtd->>zebra: NB callback: zebra_allow_external_route_update_create()
    zebra->>zebra: zrouter.allow_delete = true

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