Preserve user-supplied bracket map keys when flattening YAML

[daguimu]

Problem

YamlPropertiesFactoryBean (and any other YamlProcessor consumer) drops the literal brackets on user-supplied YAML map keys whose textual form starts with [. For example:

root:
  webservices:
    "[domain.test:8080]":
      - username: me
        password: mypassword

today flattens to:

root.webservices[domain.test:8080][0].username=me

so a downstream consumer such as Spring Boot's relaxed binder sees the map key as domain.test:8080, losing the user-typed brackets entirely.

Root Cause

YamlProcessor#buildFlattenedMap (spring-beans/.../YamlProcessor.java) treats every key starting with [ the same way:

if (key.startsWith("[")) {
    key = path + key;          // no dot separator
}

That branch is intended for internally-generated bracket keys – collection indices ([0], [1], …) and the toString() of non-CharSequence map keys (asMap line 254 wraps them as [<toString>]). When a user-supplied YAML key happens to start with [ it is silently misclassified as an internal index marker and the brackets are stripped from the resulting property path.

Fix

Distinguish the two cases by inspecting characters that never appear in internally-generated bracket keys:

• collection indices are always [<digits>]
• toString() of common non-CharSequence keys (Integer, Long, Boolean, …) contains no . or :

So, inside the key.startsWith("[") branch, when the key contains . or : it must have been written by the user. In that case the key is wrapped in an extra pair of brackets so the original brackets become part of the property path:

root.webservices[[domain.test:8080]][0].username=me

asMap and the value-tree returned to YamlMapFactoryBean users are unchanged – the additional wrapping happens only inside buildFlattenedMap, on the flatten path.

Tests Added

Change point	Test
Bracket-prefixed user key with . and : (the exact reproduction from the issue) is wrapped	loadNestedMapWithBracketedKeyContainingDotAndColon()
Bracket-prefixed user key with . only is wrapped	loadNestedMapWithBracketedKeyContainingDot()
Bracket-prefixed user key with : only is wrapped	loadNestedMapWithBracketedKeyContainingColon()
Internal collection-index keys ([0], [1], …) and Integer map keys ([1]) are unchanged	existing loadArrayOf* and integerKeyBehaves / integerDeepKeyBehaves tests still pass

All spring-beans YAML tests (YamlPropertiesFactoryBeanTests, YamlMapFactoryBeanTests, YamlProcessorTests – 43 tests) pass locally.

Impact

• YamlPropertiesFactoryBean / any YamlProcessor consumer that calls getFlattenedMap: bracket-prefixed YAML keys with . or : are now preserved with an additional bracket pair, matching what the Spring Boot binder expects for map keys with special characters.
• YamlMapFactoryBean (tree path) and existing internal bracket keys: unchanged.

Closes gh-27020