Document what a nested test project does and does not do

[davidanthoff]

Prompted by TestItemControllers.jl#97 and this discourse thread: a user put a Project.toml + Manifest.toml in test/special/, deved the package into it, added Aqua, and expected using Aqua in the test items under it to work. It fails with ArgumentError: Package Aqua not found in current path.

That is correct behaviour — the nested project supplies version pins, and Step 4 always reads the test target from the package — but nothing on the site said so. Following Step 2 rule 1 ("the innermost enclosing project of the test file") suggests the opposite, and only the Monorepo tip states the real rule, scoped to outer projects. New ### A nested project pins versions, it does not add dependencies section under Step 4, a row in the short-answer table, and a troubleshooting entry for the error text.

The nested-project pattern stays supported and is worth having: it is how different groups of test items get different pinned versions. (TestItemRuns.jl#13 fixes the half of that which was actually broken.)

Also corrected while checking the page against the implementation

• Step 2 rule 4 was wrong. "Since a package folder with its own manifest would already have been picked up in step 1, this means a package without a manifest" — step 1 searches ancestors of the file, not of the package, so a test/ folder holding both a project and a manifest reaches this path too, and the package folder's own manifest is then used.
• envContentHash was understated in four places (environments.md, test-processes.md, integrating/jsonrpc.md, integrating/language-server.md). It folds in the package's own Project/Manifest and its test/Project.toml/test/Manifest.toml as well as the chosen project's pair, so editing test dependencies does restart a pooled process.
• The generated test project also carries the package's own [deps], and resolves [targets] names through [weakdeps] as well as [extras].
• The active environment was described as a test-project fallback without the condition that it must dev the package, in integrating/language-server.md, integrating/testitemruns.md and integrating/julia-apis.md.
• pkg-test.md's setup step (Pkg.activate("test"); Pkg.add("TestItemRunner")) writes a test/Manifest.toml, which silently changes project selection for every item under test/ — now noted.

npm run docs:build passes; dead links are not ignored in the config, so the new anchors resolve.

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