GH-5166 - Apply BeforeConvertCallback result in MongoTemplate.replace()

[SUJAY-HK]

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What

MongoTemplate.replace() and ReactiveMongoTemplate.replace() call
maybeCallBeforeConvert but the returned (potentially modified) entity
is not used for BSON mapping. replaceSingleContext was already called
with the original entity before the callback fires, so the callback
result is silently discarded.

Why

The BeforeConvertCallback contract requires that the entity returned
by the callback is the one that gets mapped and persisted. The same
class of bug was previously fixed for doInsert() in #2481.

Fix

Move maybeCallBeforeConvert before replaceSingleContext in
MongoTemplate.replace() so the post-callback entity is used for
document mapping.

In ReactiveMongoTemplate.replace(), wrap the context creation and
collection operation inside .flatMap() on the callback result, since
maybeCallBeforeConvert returns Mono<T> in the reactive stack.

Tests added in MongoTemplateTests and ReactiveMongoTemplateTests
to verify the callback result is applied before the replacement is
persisted. Full test suite passes (6303 tests, 0 failures).

Closes #5166