feat(csharp): Add driver manager

[davidhcoe]

Implements C# driver manager matching adbc_driver_manager.h spec:

Core APIs:
• LoadDriver() / FindLoadDriver() - Load native/managed drivers
• OpenDatabaseFromProfile() - TOML profile support
• AdbcLoadFlags - Search control (env/user/system paths)

Features:
• Native + managed (.NET) driver loading
• TOML manifests with env_var() expansion
• Uses a custom TOML parser vs Tomlyn because a strong name is required for dependencies
• Cross-platform driver discovery
• Option merging (profile + explicit)
• Co-located manifest detection

Tests: 40+ unit tests + sample BigQuery/Snowflake implementations (attached, not checked in)

BigQueryDriverManagerTests.txt
SnowflakeDriverManagerTests.txt

[davidhcoe]

I debated whether to put this in its own assembly or as a subfolder under the shared Adbc project. I decided on a subfolder approach to reduce the number of dependencies required (especially since the AdbcDrivers are still using submodules and any tests there would require an additional submodule to be managed).

[lidavidm]

Note we noticed various inconsistencies in profiles that we're working on correcting (#4082 / #4085)

[CurtHagenlocher]

To what extent have you tried this with .NET 8.0? I'm discovering that dynamic loading of assemblies is fairly different in that environment.

[davidhcoe]

To what extent have you tried this with .NET 8.0? I'm discovering that dynamic loading of assemblies is fairly different in that environment.

It has .NET 8 tests that all pass but I added some more items in the recent push.

[davidhcoe]

Note we noticed various inconsistencies in profiles that we're working on correcting (#4082 / #4085)

I added some items to address #4082. For #4085 , based on your table, this implementation currently does:

Feature -> How It Works

• init_func (entrypoint) priority -> LoadDriver(path, entrypoint) - if provided, uses it directly
• Load driver from explicit path -> LoadDriver(driverPath)
• Load driver by name (search) -> FindLoadDriver(driverName, ...) with directory search
• Load from profile object -> OpenDatabaseFromProfile(profile, ...)
• Profile from TOML file -> TomlConnectionProfile.FromFile(path)
• Explicit options override profile -> OpenDatabaseFromProfile(profile, explicitOptions, ...)
• Managed (.NET) drivers -> LoadManagedDriver(assemblyPath, typeName)
• Native drivers -> Via CAdbcDriverImporter.Load(...)
• Co-located manifests (.toml next to .dll) -> Auto-detected in LoadDriver
• Profile/manifest version fields -> profile_version (new) and version (legacy)
• Options section naming -> [Options] (new) and [options] (legacy, case-insensitive)

❌ Not Yet Implemented

• URI parsing -> No parsing of snowflake://..., postgresql://... URIs
• Profile URI scheme -> No profile:///path/to/file.toml support
• Driver URI scheme -> No URI that embeds both driver and connection string
• Profile in options -> No automatic detection of profile key in options dict
• Multiple profile detection -> No error if profile specified in multiple ways
• Driver/profile consistency -> No validation that driver arg matches profile's driver
• URI extraction from driver -> No parsing driver argument as potential URI
• Non-driver URI detection -> No distinguishing connection URIs from driver URIs