feat: add Description field to SSA InjectedKey

[fernandezcuesta]

Description of your changes

Add a customizable Description field to InjectedKey struct. Before, SSA injected keys always got hardcoded description.

Now providers can set their own (for instance, omit the "this is an injected field with a default value..." sentence, which might be misleading) when there isn't a default value set.

Fixes #671

I have:

• Read and followed Upjet's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

Add unit test.

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds a Description field to InjectedKey, updates the builder to use that description when present (falling back to the default), and adds a unit test validating default and custom description behavior.

Changes

Configurable injected key description

Layer / File(s)	Summary
Injected key description configuration pkg/config/resource.go	InjectedKey struct adds an exported Description field, enabling custom descriptions for injected list-map keys.
List merge strategy injection with configurable description pkg/types/builder.go, pkg/types/builder_test.go	injectServerSideApplyListMergeKeys applies the configured description when present or falls back to descriptionInjectedKey. Tests verify both default and custom description handling.

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 7

✅ Passed checks (7 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely describes the main change: adding a customizable Description field to the InjectedKey struct for server-side apply operations.
Linked Issues check	✅ Passed	The PR successfully implements the objective from issue #671 by adding a customizable Description field to InjectedKey, allowing providers to set their own descriptions rather than using a hardcoded value.
Out of Scope Changes check	✅ Passed	All changes are directly related to the stated objective: adding the Description field to InjectedKey, updating the logic to use it, and adding comprehensive test coverage.
Configuration Api Breaking Changes	✅ Passed	PR only adds an optional field to InjectedKey struct in pkg/config/resource.go. No fields/types/functions removed, renamed, or signature changes—backward compatible with all existing code.
Generated Code Manual Edits	✅ Passed	No files matching the zz_*.go pattern (generated code) were modified. All PR changes are to hand-written source files (pkg/config/resource.go, pkg/types/builder.go, pkg/types/builder_test.go).
Template Breaking Changes	✅ Passed	PR modifies pkg/config/resource.go, pkg/types/builder.go, and pkg/types/builder_test.go only; no changes to pkg/controller/external*.go template files are present.
Description check	✅ Passed	The pull request description clearly explains the changes: adding a customizable Description field to InjectedKey to allow providers to override the hardcoded description text.

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@pkg/config/resource.go`:
- Around line 384-388: The InjectedKey struct's Description field lacks GoDoc;
add a GoDoc comment above Description in the InjectedKey type (reference:
InjectedKey.Description) explaining that it is the OpenAPI schema description
for the injected list-map key field, that providers may supply a custom
description, and that when empty it defaults to a standard description about
server-side apply/merge behavior so consumers understand its purpose and default
behavior.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: e431b676-2494-4051-9964-162576365f5e

📥 Commits

Reviewing files that changed from the base of the PR and between 59c4552 and 1fc5dc3.

📒 Files selected for processing (3)

• pkg/config/resource.go
• pkg/types/builder.go
• pkg/types/builder_test.go