feat(test): add comprehensive multi-database testing infrastructure

[h3n4l]

Summary

• Add multi-container support for MongoDB 4.4, MongoDB 8.0, and DocumentDB running in parallel
• Migrate all existing tests to run against all three databases using testutil.RunOnAllDBs
• Add unicode test suite covering CJK, Arabic, emoji, and special characters
• Add complex query tests (nested $and/$or, $elemMatch, cursor modifiers)
• Add BSON helper function tests (ObjectId, ISODate, NumberLong, UUID, Timestamp, etc.)
• Add fuzz tests for edge case discovery using Go's native fuzzing
• Add shared test fixtures and fixture loader utility

Test plan

• All 129 tests pass on mongo4, mongo8, and documentdb
• Fuzz tests run without panics
• Linter passes with 0 issues

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds a multi-database integration testing setup (MongoDB 4.4, MongoDB 8.0, and DocumentDB) and expands the test suite to validate behavior across engines, including unicode handling, complex queries, BSON helpers, and fuzzing.

Changes:

• Introduces multi-container test infrastructure and helpers (GetAllClients, RunOnAllDBs) plus fixture loading utilities.
• Migrates existing integration tests to run against all supported databases in parallel subtests.
• Adds new test suites (unicode round-trips, BSON helper coverage, fuzz tests) and new JSON fixtures.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
write_test.go	Migrates write-operation tests to run across all DBs via RunOnAllDBs.
error_test.go	Runs error-path tests across all DBs with per-DB database names.
database_test.go	Runs database/collection metadata tests across all DBs.
collection_test.go	Runs extensive collection/query/aggregate tests across all DBs and adds new query coverage.
unicode_test.go	Adds unicode-focused integration tests (CJK/Arabic/emoji, unicode collection names, fixture round-trip).
fuzz_test.go	Adds Go fuzz tests for filters and inserts (integration-level).
internal/testutil/container.go	Implements multi-container startup (mongo4/mongo8/documentdb) and test helper APIs.
internal/testutil/container_test.go	Adds tests validating the multi-container helper behavior and fixture loading.
internal/testutil/fixtures.go	Adds JSON fixture loader utilities for tests.
internal/translator/bson_helpers_test.go	Adds end-to-end tests for BSON helper function conversions.
testdata/users.json	Adds user fixture data.
testdata/unicode_samples.json	Adds unicode fixture data for round-trip testing.
testdata/complex_documents.json	Adds complex document fixture data for advanced query coverage.
go.mod	Promotes testcontainers-go to a direct dependency.

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