Fix nullable object initializer widening

[DavidObando]

Summary

• preserve the nullable substitution from constructed indexer properties when translating indexed collection initializers
• avoid spurious runtime !! assertions for nullable references widened into object? while retaining assertions for object/other required sinks
• cover nullable references, conditional access, generics, dictionary/collection forms, negative strict sinks, and migrated runtime behavior

Validation

• focused issue tests: 6 passed
• related cs2gs: object/collection initializer members drop oblivious external null forgiveness #2429/cs2gs: consumer taint suppresses initializer forgiveness for imported target members #2521 regression tests: 28 passed
• full Cs2Gs.Tests: 1,720 passed
• actual Oahu CLI migration: translate/compile/ILVerify/test-parity green
• fully migrated Oahu config get --json and download B000000001 --dry-run: exit 0; nullable config values preserved

Deferred work

• RC-C13 findings unrelated to nullable initializer widening (compiler metadata/ABI, ILVerify observability, and other isolated roots) remain tracked separately and are intentionally unchanged.

Fixes #2743