Fix bi-temporal union duplicate columns regression

[wendy-nweje-gs]

What type of PR is this?

• Bug Fix

What does this PR do / why is it needed ?

Problem

Bi-temporal union mappings with self-joins (e.g., Customer_Self_Join_Live) produce invalid SQL in two ways:

1. Duplicate column aliases in UNION ALL — buildUnion adds milestoning columns (e.g., "lake_from_0", "lake_thru_0") and separately the FK logic adds the same columns unquoted (e.g., lake_from_0). H2 and other databases reject duplicate aliases.

2. "Column not found" in ON clauses — modifyColumnNameInOperation computes unquoted column names (e.g., lake_thru_1) for join ON clause references, but the union defines them quoted (e.g., "lake_thru_1"). H2 uppercases unquoted identifiers, so LAKE_THRU_1 ≠ "lake_thru_1".

Testing

• New test: testBiTemporalUnionWithSelfJoin_duplicateColumnRegression — verifies no duplicate columns and correct SQL shape
• New test: testBiTemporalUnionJoin_milestoningColumnInOnClause — verifies milestoning column refs are quoted in ON clauses

Which issue(s) this PR fixes:

Fixes #

Other notes for reviewers:

Does this PR introduce a user-facing change?

[github-actions]

Test Results

  1 061 files   -        1    1 061 suites   - 1   3h 14m 11s ⏱️ - 28m 15s
19 783 tests +       6  19 604 ✔️ +       6  179 💤 ±0  0 ❌ ±0 
41 101 runs   - 1 197  40 922 ✔️  - 1 197  179 💤 ±0  0 ❌ ±0 

Results for commit 31be582. ± Comparison against base commit 96736c4.

♻️ This comment has been updated with latest results.