fix(plugin-mongodb): prevent crash on NaN or infinite document values (#1418)

[datlechin]

Problem

Opening a MongoDB collection crashes the app (SIGABRT) when a document contains a non-finite floating-point value (NaN, +Infinity, -Infinity). BSON double is IEEE-754, so these values are legal in stored documents. Reported in #1418 with a crash report from 0.44.0.

The crash trace: +[NSJSONSerialization dataWithJSONObject:options:error:] -> _writeJSONArray -> _writeJSONNumber -> objc_exception_throw -> abort().

Root cause

BsonDocumentFlattener.sanitizeForJson was not total: its default branch returned any unrecognized value unchanged, so a non-finite Double/NSNumber reached JSONSerialization.data(withJSONObject:). That method raises an Objective-C NSInvalidArgumentException for non-finite numbers (and for unsupported types). The surrounding do { try ... } catch {} was dead code: Swift cannot catch a raised NSException, only bridged NSError, so the exception unwound to abort().

MongoDBPluginDriver.prettyJson (collection validators, index key specs) had the identical uncatchable try? flaw.

Fix

• sanitizeForJson is now total: every value resolves to a guaranteed-JSON-safe form. Non-finite doubles become the tokens NaN / Infinity / -Infinity; any unsupported type becomes String(describing:) so nothing invalid can reach the writer.
• serializeToJson gates on JSONSerialization.isValidJSONObject with an early-return guard, replacing the dead catch.
• stringValue renders top-level non-finite doubles as the same tokens instead of Swift's lowercase nan/inf.
• prettyJson sanitizes and validates before serializing.

Token spellings match MongoDB Extended JSON / mongosh. Nested values render quoted ({"v":"NaN"}, valid JSON, consistent with how Data/Date are already coerced); top-level cells render bare (NaN).

Tests

Added 9 tests to BsonDocumentFlattenerTests: nested NaN/+Inf/-Inf in dicts and arrays, the flatten-level crash scenario, top-level token rendering, and an unsupported nested type that now stringifies instead of crashing.

Notes

• No PluginKit ABI change. MongoDB is registry-only, so this ships on the next plugin-mongodb-v* release after merge.
• No docs change (crash fix, not a feature/setting change).