feat(admin): make /configs/validate available in all modes

[nic-6443]

Description

The POST /apisix/admin/configs/validate endpoint currently only works in standalone (YAML) mode. This PR makes it available in etcd mode as well, so clients can perform batch configuration validation that includes plugin check_schema() advanced checks regardless of the deployment mode.

Changes

New module: apisix/admin/config_validate.lua
Extracts the shared validation logic (validate_configuration, check_conf, check_duplicate, validate handler) from standalone.lua into a reusable module.

Modified: apisix/admin/standalone.lua
Delegates to the new config_validate module, removing ~140 lines of duplicated code while keeping standalone-specific logic (get/update/head, patch_schema, config sync).

Modified: apisix/admin/init.lua
Registers POST /apisix/admin/configs/validate in the etcd mode route table, placed before the wildcard route for correct matching priority.

Response Format

Success (200): {}

Failure (400):

{
  "error_msg": "Configuration validation failed",
  "errors": [
    {
      "resource_type": "routes",
      "resource_id": "route-1",
      "index": 0,
      "error": "failed to check the configuration of plugin cors err: ..."
    }
  ]
}

Each error object includes resource_type, resource_id, index (0-based), and error (raw validator message).

Why

Clients using etcd mode (e.g., ADC adc lint, Ingress Controller, CI pipelines) had no way to perform server-side batch validation that includes advanced plugin checks (cross-field constraints, regex compilation, expression parsing, recursive sub-plugin validation, etc.). The static JSON Schema from the schema API does not cover these checks, leading to situations where local lint passes but server-side sync fails.

This endpoint accepts the same declarative config format as standalone mode and returns all validation errors at once (collect-all mode), making it suitable for lint-style workflows.

[Copilot]

Pull request overview

Enables POST /apisix/admin/configs/validate in etcd mode (in addition to standalone/YAML mode) by extracting the shared batch-validation logic into a reusable module and wiring the route in the etcd-mode Admin API router.

Changes:

• Add apisix/admin/config_validate.lua to host shared declarative-config validation logic (schema + plugin check_schema() checks, duplicate detection, collect-all errors).
• Refactor apisix/admin/standalone.lua to delegate validation to the new module.
• Register POST /apisix/admin/configs/validate in apisix/admin/init.lua so it works in etcd mode; add a new test suite.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
apisix/admin/config_validate.lua	New shared module implementing request parsing + batch validation across resource types.
apisix/admin/standalone.lua	Removes duplicated validation code and calls into config_validate.
apisix/admin/init.lua	Adds the validate endpoint route to etcd-mode router (before wildcard).
t/admin/config-validate.t	Adds coverage for the validate endpoint success/error scenarios.

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