REF-11 Harden metadata directory handling (#25, #24)

[thomasnymand]

Follow-up to #81, which was merged before these two commits landed on the branch. This PR carries the remaining metadata-directory robustness fixes.

Changes

• Allow README.TXT files in metadata directory #25 — Non-metadata files break config load. ParseFile returns null for a file that isn't SAML 2.0 metadata, but HandleCreateIdp/HandleUpdateIdp dereferenced it immediately, throwing a NullReferenceException that aborted configuration loading (e.g. a README placed in the metadata directory). Such files are now skipped gracefully; ParseFile already traces the reason. Also resolves the intermittent NRE in Metadata document missing causing NullpointerException #72.
• Improved error handling of misspelled IDPEndpoint ID #24 — Misspelled default IDPEndpoint id. A misspelled id leaves the configured endpoint's metadata null; when selected as the default, RetrieveIDP returned it and the caller hit an opaque NullReferenceException. Now throws a descriptive Saml20Exception naming the id, reusing the previously-unused MetadataNotFound resource (reworded to point at the likely misspelling).

Notes

• The Improved error handling of misspelled IDPEndpoint ID #24 check is placed at point-of-use (RetrieveIDP) rather than in IDPEndpoints.Initialize(): the test config intentionally declares endpoints without matching metadata files, and Initialize() runs on every config load, so a load-time throw would break the test suite.

Testing

CI (build-and-test) passing on the head commit. No cross-platform build path exists locally (Windows/msbuild only).

🤖 Generated with Claude Code