[repo-assist] fix: surface FSI evaluation errors to stderr by default in FsiEvaluator

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Closes #685

Problem

When a code snippet fails during fsdocs --eval (e.g., native library loading fails), the error was silently discarded. The FsiEvaluator catches FsiEvaluationException and fires the EvaluationFailed event, but that event has no default subscribers. The onError callback also defaults to ignore. This meant that users saw empty output blocks with no indication that evaluation had failed — exactly the symptom described in #685.

Fix

Always write a concise error message to stderr when FSI evaluation fails, regardless of whether the caller subscribed to EvaluationFailed or provided an onError callback. The message includes the file name and the FSI stderr output, which typically contains the actual error (e.g., the NativeLibrary.Load failure message).

The onError callback (used by the --strict pipeline) continues to work as before. The misleading "and --strict is on" phrase in the error message has also been removed — it was displayed even when --strict was not in effect.

Root Cause

onError defaults to ignore:

let onError = defaultArg onError ignore

And the EvaluationFailed event has no subscribers in the fsdocs pipeline. So evaluation failures were silently swallowed.

Trade-offs

• The stderr message is always emitted, even in non-strict mode. This is intentional — invisible failures are worse than noisy ones. Users who want silence can redirect stderr.
• No breaking API change: existing subscribers to EvaluationFailed and onError callbacks still work.

Test Status

✅ Build succeeded (dotnet build — Release)
✅ 143/143 tests passed (tests/FSharp.Literate.Tests)

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