Accept ActiveDirectoryManagedIdentity as an Authentication alias (don't perpetuate msodbcsql's ActiveDirectoryMSI-only quirk)

[saurabh500]

Problem statement

In review of #107, @David-Engel flagged the Authentication keyword-value handling in mssql-odbc/src/connection/connection_string_parser.rs:

I hate that we are perpetuating this mistake. 😢

No need to change it here since the PR is intentionally replicating msodbcsql behavior. Just sharing disappointment. Maybe we can fix it in the future...

The "mistake": msodbcsql only accepts ActiveDirectoryMSI as the Authentication value for managed-identity auth, and rejects the intuitive, self-documenting spelling ActiveDirectoryManagedIdentity — even though both map to the exact same auth method (dlgattr.h). Because #107 intentionally mirrors msodbcsql byte-for-byte, mssql-odbc inherits this:

Server=h;Authentication=ActiveDirectoryManagedIdentity   -> ERROR (rejected)
Server=h;Authentication=ActiveDirectoryMSI               -> OK

(See the authentication_managed_identity_is_rejected test, ~line 825, and referenced bug #46177.)

This is a poor experience: ActiveDirectoryManagedIdentity is the descriptive name users know from other Microsoft data drivers/SDKs, and having a brand-new driver reject it in favor of the cryptic ...MSI abbreviation perpetuates a long-standing wart with no back-compat reason to keep it.

Proposed solution

In mssql-odbc, accept ActiveDirectoryManagedIdentity as an alias for the managed-identity auth method, in addition to ActiveDirectoryMSI, so both resolve to the same method. Keep ActiveDirectoryMSI working for compatibility. Update the parser's recognized-keyword set, the authentication_managed_identity_is_rejected unit test (it should become "accepted"), and mssql-odbc/docs/connection_string_parser.md.

This is an intentional, documented divergence from msodbcsql in favor of correctness/UX — the same class of decision tracked in the broader parity-review follow-up (#114). Consider handling it there or as a focused change.

Affected crate

mssql-odbc (not in the dropdown; closest: Not sure / Multiple)

Alternatives considered

• Keep strict msodbcsql parity (status quo from Rewrite ODBC connection string parser to mirror msodbcsql ParseAttrStr #107): reject ActiveDirectoryManagedIdentity. Rejected — this is exactly the behavior David called a "mistake" we shouldn't perpetuate.
• Rename to only accept ActiveDirectoryManagedIdentity and drop ActiveDirectoryMSI: rejected — would break users/scripts already relying on the ...MSI spelling. Accept both.

Additional context

• Source: PR Rewrite ODBC connection string parser to mirror msodbcsql ParseAttrStr #107 review comment by @David-Engel (id 3589966032), anchored to the authentication_managed_identity_is_rejected test in connection_string_parser.rs.
• Related: bug #46177 (referenced in the test comment), and the broader parity-review follow-up Reconsider byte-for-byte msodbcsql parity for harmful connection-string parsing quirks (mssql-odbc) #114.