Refactor: centralize resource type and plan field identifiers

[rspurgeon]

Summary

Stage 1 of the cleanup requested in #792.

This PR centralizes declarative resource type and planner/executor field identifiers so the declarative pipeline no longer depends on scattered string literals for plan contracts and resource references.

What Changed

• Added planner field constants for common plan fields, relationship fields, portal fields, and event-gateway fields.
• Aliased planner resource type constants to the canonical identifiers in internal/declarative/resources.
• Changed resources.ResourceRef.Kind from string to resources.ResourceType.
• Replaced planner and executor resource-type literals with constants across declarative planning, execution, state error context, dependency resolution, and progress/prompt formatting.
• Replaced required-field and plan-field map key literals with shared planner.Field* constants where those strings are part of planner/executor contracts.

Reviewer Notes

This is intentionally mechanical and limited to the declarative resource/planner/executor boundary. It does not include the separate kongctl view cleanup, which should remain a later stage.

I left domain payload keys as literals where they represent API/config subdocuments rather than planner contract fields, for example OIDC config keys, menu item keys, schema registry config paths, and selector payload keys.

Validation

• make build
• make lint
• make test

Closes #792

[Copilot]

Pull request overview

This PR is stage 1 of #792, refactoring the declarative planner/executor boundary to centralize resource type identifiers and plan contract field keys (reducing scattered string literals across the pipeline).

Changes:

• Centralized planner/executor contract keys via planner.Field* constants (common fields, relationships, portal fields, and event-gateway fields).
• Centralized resource type identifiers by aliasing planner resource types to canonical internal/declarative/resources identifiers.
• Updated resources.ResourceRef.Kind from string to resources.ResourceType and propagated type-safe usage across planners/executors and tests.

Reviewed changes

Copilot reviewed 108 out of 108 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/declarative/resources/types.go	Adds new resource types and makes ResourceRef.Kind typed (ResourceType) instead of raw string.
internal/declarative/resources/*.go (+ tests)	Replaces dependency kind string literals with ResourceType* constants and updates tests accordingly.
internal/declarative/planner/constants.go	Introduces shared Field* constants and aliases ResourceType* to canonical resource identifiers.
internal/declarative/planner/*.go	Replaces plan field/resource type string literals with shared constants across planning, dependency resolution, and protection logic.
internal/declarative/executor/*.go	Replaces executor-side plan field/resource type string literals with shared constants (adapters, progress formatting, deck execution).
internal/declarative/common/*.go	Updates helper logic and prompts to reference shared planner field constants.
internal/declarative/errors/api_errors.go	Updates conflict/validation hint matching to use canonical resource type identifiers.
internal/declarative/protection/lookup.go	Adds new resource types to the managed/protected resource type set.