Support TTL indexes in the GORM-Mongo mapping DSL and reconcile index option changes

[codeconsole]

What

Two related improvements to MongoDB index initialisation in MongoDatastore.initializeIndices.

1. TTL indexes can be declared in the mapping DSL

A single-field index declared with expireAfterSeconds now materialises a MongoDB TTL index:

static mapping = {
    created index: true, indexAttributes: [expireAfterSeconds: 7776000]   // 90 days
}

Previously this was impossible. MongoConstants.mapToObject builds IndexOptions by reflectively invoking same-named single-argument setters, but the driver's only TTL setter is the two-argument IndexOptions.expireAfter(Long, TimeUnit), which the mapper can't reach. So expireAfterSeconds is now pulled out of the index attributes and applied explicitly.

TTL is single-field only — MongoDB rejects expireAfterSeconds on a compound index with CannotCreateIndex; that error surfaces unchanged.

2. Index-option conflicts are reconciled, not just logged

All three index paths (declared getIndices(), getCompoundIndices(), per-property) now route through one helper. On IndexOptionsConflict (server error 85 — an index already exists on these keys with different options) it reconciles instead of only logging the stack trace:

• TTL value change (e.g. a configurable retention changed between restarts) is applied in place via collMod — no drop, no rebuild, no window without an index.
• Any other conflicting change is dropped and recreated only when indexAttributes: [recreateOnConflict: true] is declared; otherwise it logs a clear, actionable message. A unique index is never silently dropped.

Why

Without DSL support, a TTL index has to be created imperatively (raw-driver createIndex in a bootstrap hook), and changing its retention needs hand-rolled drop-and-recreate logic. That is easy to get wrong: a stale plain index on the same key blocks the TTL create, and the TTL then silently never applies. Declaring it in the mapping, with in-place reconciliation handled by the framework, removes both the boilerplate and that footgun.

Tests

TtlIndexSpec (runs against the Mongo testcontainer) covers:

• expireAfterSeconds in the mapping produces a TTL index with the declared value.
• Mutating the live TTL and re-initialising restores the declared value on the same index (one index, no conflict) — exercising the collMod reconcile path.

Existing IndexAttributesAndCompoundKeySpec (unique / compound) still passes.

[testlens-app]

✅ All tests passed ✅

🏷️ Commit: b939e01
▶️ Tests: 40374 executed
⚪️ Checks: 44/44 completed

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.0000%. Comparing base (c64ab81) to head (b939e01).
⚠️ Report is 7 commits behind head on 8.0.x.

Additional details and impacted files

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##   8.0.x   #15747   +/-   ##
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