Adds a deterministic pre-check on top of htsh's exception-based fallback:
before calling /content/abstract or /content/overview on a non-pseudo URI,
probe /api/v1/fs/stat. If the server says the URI is a file, route straight
to /content/read instead of eating a failing 500 round-trip.
This is the same idea pty819 and chennest independently landed in PRs
#12757 and #12937 — merged here on top of htsh's broader fix so we keep
pseudo-URI normalization and v0.3.3 browse-shape handling while avoiding
the slow exception path on servers that return a raised 500 every time.
The exception fallback from #5886 stays in place for environments where
fs/stat is unavailable or returns an unfamiliar shape.
Also credits pty819, chennest, and htsh in AUTHOR_MAP so future release
notes attribute them correctly.
OpenViking returns 500 for /content/abstract and /content/overview when URI points to mem_*.md files.
Add resilient fallback to /content/read for non-pseudo summary file URIs while preserving pseudo summary normalization.
Also add regression tests for fallback behavior.
