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.
OpenViking v0.3.3 expects directory URIs for abstract/overview reads.
Passing pseudo-files like /.overview.md and /.abstract.md to
/api/v1/content/overview|abstract triggers HTTP 500.
This change normalizes those pseudo-URIs to their parent directory for
abstract/overview requests, preserves full reads, and hardens parsing for
wrapped/unwrapped result payloads and fs list response shapes.
hitesh