feat(sqlserver): composite FK aggregation via STRING_AGG / FOR XML PATH (#146)

[saurabh500]

Closes #146.

What

Aggregate composite foreign-key columns into a single result row per constraint, instead of one row per column. Adds STRING_AGG (WITHIN GROUP ...) for SQL Server 2017+ and a STUFF(... FOR XML PATH(''), ...) fallback for 2012-2016, dispatched at runtime via a new detect_server_version probe against SERVERPROPERTY('ProductVersion') (with @@VERSION and a DEFAULT_MAJOR = 14 fallback).

Changes

• models.rs — ForeignKey gains 3 #[serde(default)] fields (columns: Vec<String>, ref_columns: Vec<String>, ref_schema: Option<String>), plus local_columns() / referenced_columns() helpers that fall back to the legacy single-column fields. Old persisted JSON deserializes unchanged.
• drivers/sqlserver/introspection.rs
  • Replaces Q_GET_FOREIGN_KEYS / Q_GET_ALL_FOREIGN_KEYS_BATCH with _STRING_AGG / _XML_PATH variants. Both branches emit the same row shape: name, ref_schema, ref_table, columns, ref_columns, on_update, on_delete (+ table_name for the batch variants). Cascade actions are normalised in SQL via CASE over sys.foreign_keys.update_referential_action so the output matches the INFORMATION_SCHEMA form the rest of the codebase already expects (NO ACTION, not NO_ACTION).
  • build_foreign_key signature now takes column arrays; legacy column_name / ref_column are derived from the first array element so downstream consumers (FK list view, ER diagram) keep working before they migrate.
  • get_foreign_keys / get_all_foreign_keys_batch detect the server version once per call, pick the right query, and parse the comma-aggregated columns via a new split_agg_columns helper.
  • New detect_server_version async helper reuses the existing parse_major_version / parse_version_banner / DEFAULT_MAJOR from version.rs.
• drivers/{sqlite,mysql,postgres}/mod.rs — 6 FK construction sites get ..Default::default() so new fields stay at their default.

Tests

• Updated unit tests for query shape on both _STRING_AGG and _XML_PATH variants and both single-table and batch flavours (asserts STRING_AGG WITHIN GROUP, STUFF(... FOR XML PATH('')), deterministic ORDER BY fkc.constraint_column_id, normalised action strings).
• New build_foreign_key tests covering: composite columns, empty arrays, missing actions.
• split_agg_columns parser tests.
• ForeignKey::local_columns() / referenced_columns() fallback test for drivers that haven't migrated.
• Regression test deserialising the legacy JSON shape (no columns / ref_columns / ref_schema) — verifies serde defaults still kick in.
• cargo test --lib drivers::sqlserver — 103 passed, 0 failed.
• cargo test --lib — 608 passed, 2 pre-existing flaky updater::tests failures (Windows flatpak/snap detection) unrelated to this PR.

[saurabh500]

@debba this is ready for review