fix(plugin-mongodb): build the export header from a field census, not the first 200 documents

[datlechin]

Summary

• MongoDB exports (every format, plus Object Copy and Compare, which read the same stream) drop every top-level field that first appears after the 200th document. Root cause: the stream fixes its header from the first 200 documents and projects every later document onto it, so the header now also takes the fields a server-side census ($objectToArray, $unwind, $group) finds in the documents the stream reads, grouped by an object keyed on the field name so no collation can merge Email with email, and a stream whose census cannot run logs the fields it left out.
• A field that is null in all of the first 200 documents is exported as VARCHAR, so MQL export writes its later dates, ObjectIds and decimals as plain strings. Root cause: the same 200-document sample fixes each column's type, so a sampled column that held only nulls now takes the type the census found.

Tests

• MongoStreamProjectionTests/censusFieldJoinsTheHeader: 200 sampled documents without late and a census reporting late as a date give a late column typed TIMESTAMP and a non-null cell for document 201. Fails without the fix (columns ["_id", "name"]).
• MongoStreamProjectionTests/censusFieldsFollowTheSampledColumns, censusFieldTakesTheMajorityType, censusTypesAColumnTheSampleHeldOnlyNullsIn: census field order, majority typing with nulls not voting, and a sampled all-null column typed by the census. Each fails without the fix.
• MongoStreamProjectionTests/fieldsOutsideTheHeaderAreReported, fallbackHeaderAnnouncesOnlyTheId: the fields a header left out are reported. They do not compile without the fix.
• MongoFieldCensusTests/fieldNamesAreGroupedAsObjectKeys, groupRepliesBecomeTallies, missingExampleIsNull: fields are grouped by an object keyed on the field name and read back from the group's name. All three fail with a {name, type} group key. Checked on MongoDB 5.0, 6.0, 7 and 8.2: under a collection collation of {locale: "en", strength: 2} and a statement collation of {locale: "en", strength: 1, alternate: "shifted"}, a {name, type} key returns one group for Email/email and for first_name/firstname, and the object key returns both names.
• MongoFieldCensusTests (the other cases): the census covers the documents the stream reads (a find's filter, paging, row ceiling and projection; an aggregation's own output; no census for $out or $merge) and keeps the statement's hint, collation and maxTimeMS.
• MongoStreamProjectionTests/missingCensusKeepsTheSampledColumns, censusLeavesSampledTypesAlone: without a census the header and its types are unchanged.
• Covering run, 215 of 215 passed: MongoStreamProjectionTests, MongoFieldCensusTests, BsonDocumentFlattenerTests, BsonFieldPathArrayTests, BsonFieldPathTests, MongoDBCollectionSchemaTests, MongoDBNestedValueWriteTests, MongoScriptCommandBuilderTests, MongoScriptCursorOptionsTests, MongoScriptJsonTests.

Docs

The docs rewrite branch covers the user-facing text.

CI fixes

The Package Tests job failed in SyncRecordMapperTests.unknownWireValueFailsClosed: an unknown Safe Mode value decoded as Off instead of Confirm Writes. The sync mapper now uses the model's SafeModeLevel(wireValue:isReadOnly:) decoder. Unknown values require confirmation, legacy read-only restrictions remain, and renaming a connection preserves its unknown wire value. Added regression tests for read-only restrictions and renames.

Integrated main and fixed another inherited build error: SurrealValue.text(_:) called jsonText(length), but main had replaced that helper with a property. Restored the length-aware helper, keeping JSONTruncation for grid display and full JSON for exports. Existing long-structure export and grid-display tests cover both paths.

Follow-up validation on the final merged branch:

• swift test --package-path Packages/TableProCore: PASS, all Swift Testing and XCTest runs passed.
• All SurrealDB suites in an isolated SwiftPM harness: PASS. The harness compiles the actual plugin source files, the existing SurrealDBDriverTests.swift suites, and the host's TableFilter model where needed. It adapts imports for the separate test module; it does not run the app's Xcode test host.
• SwiftLint on changed Swift files: zero violations.
• Project generation and git diff --check: PASS.
• verify.sh plugins (AllPlugins): PASS on the merged fix(plugin-surrealdb): BETWEEN, REGEX and IS EMPTY filters become equality checks #3218 branch, including the common SurrealDB helper fix.
• The lint helper's separate documentation check reports four existing stale paths in CLAUDE.md and the verification guide, which this follow-up leaves unchanged.
• Reviewed the final diff directly.
• GitHub CI reruns pending after push.