[Feature] Give test suite nodes a source location

### Request Overview

Written with the help of AI; reviewed by me 😉

With `cmake.ctest.testSuiteDelimiter` set, tests are grouped into suite nodes in the Test
Explorer. The leaf tests get a source location — we supply `DEF_SOURCE_LINE` — so they show run
decorations in the editor gutter and "Go to Test" works. Suite nodes have no location, so neither
does.

Running a suite already works from the panel, so all that seems to be missing is the location: a
`TestItem` with a uri and range gets a gutter decoration from VS Code, and activating it goes
through the same run handler, which already expands a suite to its leaf tests via `flattenTests`.
So a suite would become runnable from the editor for free.

The catch is that suite nodes are synthesised by splitting test names, so there's no obvious place
for a location to come from. One idea: if a registered test's name is exactly the prefix that
would otherwise become a synthesised suite — a test named `unit.GgaSentenceTests` alongside
`unit.GgaSentenceTests.parses_a_valid_case` — use that test *as* the suite node, taking its
`DEF_SOURCE_LINE`. That needs no new property, and it would also remove the duplicate entry
reported in #4408, where such a test currently appears as a sibling of the suite instead of being
it.

### Additional Information

Suite items are created without a uri today, while the leaf item just below gets one
(`createTestItemAndSuiteTree` in src/ctest.ts):

```ts
suiteItem = initializedTestExplorer.createTestItem(suiteId, suiteId);
...
testItem = initializedTestExplorer.createTestItem(testName, testLabel, uri);
```

`TestItem.uri` is readonly, so the location has to be known at creation time.

Worth flagging: falling back to the location of the suite's first child wouldn't help. The
decoration would land on the first test case's line instead of the suite declaration, so you'd get
two decorations on one line and still nothing on the suite. Only the suite's own line is useful.

For context we're on Boost.Test, where this maps cleanly. `--list_content=DOT` reports file and
line for suites as well as cases, and `--run_test=` runs a whole suite, so we can register
`unit.GgaSentenceTests` with a correct `DEF_SOURCE_LINE` and a working command today — it just
ends up beside the suite node rather than becoming it. Happy to test a change against a real
project.

Start in src/ctest.ts at createTestItemAndSuiteTree, where suite and leaf TestItems are created, and trace how DEF_SOURCE_LINE and flattenTests are used. Use a registered test whose name matches a synthesized suite as that suite's source location; done means the suite has the correct uri and range, runs from the editor, and no duplicate sibling remains as described in #4408.

Written by the indexing model from the issue text.