docs: mention jhack scenario snapshot

[dimaqq]

Fixes #1957

[Copilot]

Pull request overview

Updates the documentation to mention using jhack scenario snapshot as a way to capture realistic live model/relation data and convert it into ops.testing state-transition tests, addressing #1957.

Changes:

• Add a short note in the unit-testing how-to about capturing live relation inputs via jhack scenario snapshot.
• Add a new section in the relations how-to describing a workflow for snapshotting live relation state and turning it into local tests.
• Add a note in the state-transition testing explanation linking to the relations workflow.

Reviewed changes

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

File	Description
docs/howto/write-unit-tests-for-a-charm.md	Adds guidance pointing readers to jhack scenario snapshot for realistic relation test inputs.
docs/howto/manage-relations.md	Introduces a new subsection describing snapshot-to-test workflow for relation debugging.
docs/explanation/state-transition-testing.md	Adds a short pointer to the relations workflow for realistic starting data.

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[Copilot]

The new "### Capture live relation state for testing" heading is the same level as "### Add code to use a relation", which leaves the "Add code..." section empty and also makes the existing "#### Using a charm library" subsection fall under the new testing section. Consider making "Capture..." a subsection under "Add code..." (e.g., use "####") or move it to a more appropriate place so the heading hierarchy matches the content.

Suggested change

	### Capture live relation state for testing
	#### Capture live relation state for testing

[dimaqq]

This is valid, there's probably a better place to mention jhack than the very beginning of "use a relation"

[dwilding]

I recommend putting the new section here:

• Test the feature
  • Write unit tests
  • Write integration tests
• Capture live relation state for testing

Alternative title idea: Generate tests from a deployed model

[dwilding]

TIL jhack scenario snapshot 🙂

[dwilding]

I recommend putting the new section here:

• Test the feature
  • Write unit tests
  • Write integration tests
• Capture live relation state for testing

Alternative title idea: Generate tests from a deployed model

[dimaqq]

Updated and ready for real review.