refactor: add type-safe BSON helpers in writer_security

[retran]

Why

writer_security.go contains ~26 type assertions on BSON interface values. Silent comma-ok patterns (v, _ = f.Value.(string)) swallow unexpected types and produce zero-value strings/bools that corrupt security data without any diagnostic. Bare assertions (ref = mf.Value.(string)) panic at runtime on type mismatches. Both patterns make debugging serialization issues extremely difficult. Centralizing these into typed helpers adds diagnostic logging and eliminates the entire class of silent-corruption and panic-crash bugs.

Summary

• Add bsonString and bsonBool helper functions that extract typed values from BSON interface fields with diagnostic logging for unexpected types
• Replace all 20 silent comma-ok type assertions (v, _ = f.Value.(string)) and 6 bare panic-risk assertions (ref = mf.Value.(string)) with helper calls
• Zero silent comma-ok primitive assertions remain in writer_security.go (bson.D/bson.A assertions with proper if !ok guards left unchanged)

Stacked on

• refactor: introduce LintReader interface, decouple linter from sdk/mpr #5 (pr2-reader-migration) → refactor: quick-win cleanup — panic stubs, SaveToFile, dead code, error handling, syncWriter #4 (pr1-quick-wins)

Test

make build && make test && make lint-go — all pass

[Copilot]

Pull request overview

Refactors sdk/mpr/writer_security.go to centralize BSON primitive type extraction into small helpers, eliminating silent comma-ok assertions and panic-prone type assertions while adding diagnostics for unexpected BSON value types.

Changes:

• Added bsonString / bsonBool helpers to safely extract typed values from BSON fields with logging on type mismatches.
• Replaced multiple silent v, _ := ... assertions and bare .(type) assertions with helper calls across security-writer mutations/merges.

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[retran]

Superseded by upstream PR mendixlabs#297