getMap error message shows null value for hyphenated or mixed-case property names

Describe the bug
DefaultConfigProperties.getMap(String) builds the "Invalid map property" error message with the raw property name, so the value shown is null whenever the caller passes a name that needs normalization (hyphens or upper case).

Steps to reproduce
Call getMap with a hyphenated name whose value is a malformed map entry:

DefaultConfigProperties.createFromMap(Collections.singletonMap("test-map", "a=1,b"))
    .getMap("test-map");

What did you expect to see?
A message reporting the actual configured value, e.g. Invalid map property: test-map=a=1,b, matching the value lookup that getList already does with ConfigUtil.normalizePropertyKey.

What did you see instead?
Invalid map property: test-map=null. The stored config keys are normalized (test-map -> test.map), but DefaultConfigProperties.getMap() looks the value up with the un-normalized name via config.get(name), so the lookup misses and prints null.

What version and what artifacts are you using?
Artifacts: opentelemetry-sdk-extension-autoconfigure-spi
Version: main @ 09d6c17f3
How did you reference these artifacts? Gradle, io.opentelemetry:opentelemetry-sdk-extension-autoconfigure-spi

Environment
Compiler: Temurin 21
OS: N/A

Additional context
Only the error message string is affected; getMap's normal return path already uses the normalized key, so hyphenated or mixed-case names work otherwise.

Start at DefaultConfigProperties.getMap(String) and compare its error-message lookup with the normalized-key lookup used by getList. Reproduce the malformed "test-map" case through createFromMap, then verify that the error reports the configured value rather than null while normal getMap behavior remains unchanged.