lsp: the test diagnostic pass reads the sibling defaults file once per problem

Problem

On main after #1266 merged (verified at 6c152e3), diagnoseTestPublications calls sourceForTestDiagnostic inside its per-problem loop (pkg/flowstate/v1/flowfile/lsp/testfile.go, the loop over problems.Problems), and that falls through to readCalleeSource(path) for every problem whose owning URI is not the suite and not an open defaults buffer — with no memoization. testProblemRange then re-splits the returned source into lines per diagnostic.

Measured during the #1266 review with a temporary counter on readCalleeSource: a suite whose sibling testdefaults.yaml declares 30 bad defaults.inputs entries produced 20 file reads for a single publication — one per problem, capped only by MaxLoadProblems. Worst case is 20 × maxDocumentBytes of file I/O plus 20 whole-file line splits per publication, and the publication fires on every keystroke in any open suite whose directory has a broken testdefaults.yaml that is not itself open, and once per suite at didOpen (33 open suites → 660 reads).

This crosses the rule written on the function being called — callinputs.go: "The reads are on an explicit completion or hover request rather than on every keystroke of a diagnostic pass" — and AGENTS.md invariant 5, since both the file size and the problem count are controlled by the workspace's author.

Desired outcome

One publication of a suite's diagnostics reads any given sibling source at most once, however many problems it owns, and converts positions from a line slice split once per file.

Acceptance criteria

• A publication with N sibling-owned problems performs one read of that sibling (assertable with a counter or a counting filesystem seam, the way the review measured it).
• testProblemRange (or its replacement) receives pre-split lines rather than re-splitting per diagnostic.
• Existing loader-position tests stay green.

Raised in the Claude review on #1266 inside the merge window; filed so it does not sit in a closed PR's threads.

Start in pkg/flowstate/v1/flowfile/lsp/testfile.go, tracing diagnoseTestPublications, sourceForTestDiagnostic, readCalleeSource, and testProblemRange; compare the read rule in callinputs.go and invariant 5 in AGENTS.md. Verify that one publication reads each sibling source once, splits its lines once, and keeps the existing loader-position tests green.