fix: preserve Nacos config file extension across refresh

[daguimu]

Problem

When a Nacos config change is pushed to the client, NacosPropertySourceRefreshListener#handle(NacosConfigRefreshEvent) rebuilds the affected NacosPropertySource by calling:

nacosPropertySourceBuilder.build(dataId, group, "properties", prevpropertySource.isRefreshable());

The third argument — the file extension — is hard-coded as "properties". NacosDataParserHandler#parseNacosData then selects PropertiesPropertySourceLoader regardless of the user's actual format (yaml, yml, json, xml). So after the first refresh, a yaml config like

server:
  port: 8080

is re-parsed as a properties file and produces a single bogus key server:\n port with value 8080, replacing the correctly-parsed yaml source that was installed at startup.

Root Cause

NacosPropertySource never remembers the extension it was created with. At refresh time the listener therefore has no way to recover the original format and defaults to "properties". This affects both the main spring.cloud.nacos.config.file-extension as well as extension-configs / shared-configs that may declare their own suffix per entry.

Fix

• Add an optional fileExtension field to NacosPropertySource with a new public constructor overload and a getFileExtension() accessor. The existing constructors stay for backward compatibility.
• NacosPropertySourceBuilder#build already receives the file extension from its callers, so it now passes it into the new constructor.
• NacosConfigDataLoader#doLoad propagates NacosItemConfig#getSuffix(), which is already the per-entry suffix, so extension-configs/shared-configs with non-default formats are also preserved.
• NacosPropertySourceRefreshListener#handle reads the preserved extension from the previous NacosPropertySource, falls back to NacosConfigProperties#getFileExtension() (useful when the source was built via the legacy constructor from third-party code), and finally keeps the historical "properties" default so nothing regresses.

Tests Added

Change Point	Test
Legacy constructor leaves fileExtension unset	NacosPropertySourceTest#fileExtensionIsNullWhenUsingLegacyConstructor
New constructor preserves the provided extension	NacosPropertySourceTest#fileExtensionIsPreservedWhenProvided, otherPropertiesAreNotAffectedByNewConstructor
New constructor accepts null without side effects	NacosPropertySourceTest#fileExtensionCanBeNullExplicitly
Builder#build propagates the yaml extension into the resulting NacosPropertySource	NacosPropertySourceBuilderTest#builtPropertySourcePreservesFileExtension
Builder#build propagates the properties extension	NacosPropertySourceBuilderTest#builtPropertySourcePreservesPropertiesExtension

Impact

• Yaml / yml / json / xml Nacos configs retain their correct parser across pushes — the refresh path now mirrors the initial-load path.
• Properties users see no behavioral change: their extension is preserved explicitly instead of implicitly re-defaulting.
• Third-party code that still calls the legacy NacosPropertySource constructor keeps working; the refresh listener's two-step fallback covers that case.
• Scope is limited to spring-alibaba-nacos-config; no public contract on NacosPropertySource was removed.

Addresses #4296