Allow editing values_ in status_item and small refactor

[MahmoudAlmasri]

This PR allows editing values- within status_item in a convenient way:
* Add new constructor that takes an extra vector of KeyValue
* Introduce the addValue() method to StatusItem.
Finally, a small refactor is performed to reuse some code.

[ct2034]

Thanks for your contribution. Could you please add a unit test for these functions.

[Copilot]

Pull request overview

Adds a small extension to diagnostic_aggregator::StatusItem so callers can initialize and edit the internal values_ (KeyValue pairs) more conveniently, and introduces a focused unit test for this behavior.

Changes:

• Added a new StatusItem constructor that accepts an initial std::vector<diagnostic_msgs::msg::KeyValue>.
• Added StatusItem::addValue() (insert-or-update) and refactored hasKey() to reuse shared lookup logic.
• Added a new gtest target to validate constructor/value-editing behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
diagnostic_aggregator/test/test_status_item.cpp	New gtest coverage for the values-initializing constructor, addValue(), and hasKey().
diagnostic_aggregator/src/status_item.cpp	Implements the new constructor, addValue(), hasKey(), and internal findKey().
diagnostic_aggregator/include/diagnostic_aggregator/status_item.hpp	Declares the new constructor and methods; moves hasKey() out-of-line and adds findKey() declaration.
diagnostic_aggregator/CMakeLists.txt	Registers the new gtest target and links it against the library.

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[ct2034]

Just some small refinements

[ct2034]

Suggested change

	EXPECT_EQ(item.getValue("a"), "1");
	EXPECT_EQ(item.getValue("b"), "2");
	EXPECT_EQ(item.getValue("a"), "1");
	EXPECT_EQ(item.getValue("b"), "2");
	EXPECT_TRUE(item.hasKey("a"));
	EXPECT_TRUE(item.hasKey("b"));
	EXPECT_FALSE(item.hasKey("c"));

[ct2034]

Suggested change

	item.addValue("k", "new");
	EXPECT_EQ(item.getValue("k"), "new");
	EXPECT_EQ(item.getValue("k"), "old");
	item.addValue("k", "new");
	EXPECT_EQ(item.getValue("k"), "new");

[ct2034]

just a though: I think a size() method would also be helpful

[ct2034]

@MahmoudAlmasri Could you please explain the rationale for this feature?
Why do you not edit the values in diagnostic_msgs::msg::DiagnosticStatus directly?

[MahmoudAlmasri]

@ct2034 Conceptually, a StatusItem allows reading values and emitting a path-prefixed, staleness-aware message, but until this PR, there was no way to populate values except by copying them in from a wire DiagnosticStatus. This PR closes that asymmetry.
The existing alternative is DiagnosticStatusWrapper. But using it would demand to depend on diagnostic_updater package where it lives.
Thanks to the introduced feature, the user would gain more freedom on safely editing the status item without this extra dependency.