Fix async redirect handling regression from FOLLOW_REDIRECTS default

[cursor]

Restore async-mode default that disables automatic HTTP redirect following unless FOLLOW_REDIRECTS is explicitly set.

Fixes

• Async connectors (e.g. Slack Analytics) that rely on intercepting 3xx responses to fetch pre-signed download URLs

Bug and impact

Commit #1278 introduced FOLLOW_REDIRECTS but changed the default from !processingContext.getAsync() to true. Async connectors without an explicit FOLLOW_REDIRECTS=FALSE env var (Slack Analytics has empty environment_variables) now have the HTTP client follow redirects transparently. The proxy never sees the 3xx + Location header, so the async redirect-handling block in ApiDataRequestHandler is bypassed and sanitized data is not written to the async output bucket.

Root cause

ApiDataRequestHandler defaulted setFollowRedirects(true) when FOLLOW_REDIRECTS is unset, replacing the prior behavior that disabled redirect following in async mode.

Fix and validation

• Restore default: orElse(!processingContext.getAsync())
• Update ProxyConfigProperty javadoc to document the async/sync default split
• Add asyncModeDisablesRedirectFollowingByDefault unit test
• Ran mvn -pl core -am test -Dtest=ApiDataRequestHandlerTest#asyncModeDisablesRedirectFollowingByDefault,ApiDataRequestHandlerTest#handleShouldFollowRedirectManuallyInAsyncMode

Change implications

• dependencies added/changed? no
• something important to note in future release notes?
  • Fixes Slack Analytics async file downloads on v0.6.6 when FOLLOW_REDIRECTS is not explicitly configured
  • breaking changes? no