fix(chroma): handle collection not found exception during initialization

[zksite]

Fix collection existence check broken by error message change in Chroma API.

The Chroma backend recently changed its error message from:
"Collection [...] does not exists"
to:
"Collection [...] does not exist"

This broke the string-based detection in getCollection(), causing 404s to be treated as unexpected errors and preventing initialization.

This PR replaces fragile string matching with reliable HTTP 404 status code checking via HttpClientErrorException.NotFound.

[Copilot]

Pull Request Overview

This PR fixes a bug in the Chroma vector store initialization where the collection existence check broke due to an upstream API error message change. The fix replaces fragile string-based error detection with robust HTTP 404 status code checking.

Key Changes:

• Replaced string matching on error messages with HTTP status code checking using HttpClientErrorException.NotFound
• Added proper exception handling with try-catch block to differentiate between "collection not found" (404) and genuine errors

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[Copilot]

The exception handling logic checks ex.getCause() for HttpClientErrorException.NotFound, but if chromaApi.getCollection() directly throws HttpClientErrorException.NotFound (not wrapped), this check will fail and re-throw the exception incorrectly. Consider also checking if ex itself is an instance of HttpClientErrorException.NotFound before checking the cause: if (!(ex instanceof HttpClientErrorException.NotFound || ex.getCause() instanceof HttpClientErrorException.NotFound))

Suggested change

	if (!(ex.getCause() instanceof HttpClientErrorException.NotFound)) {
	if (!(ex instanceof HttpClientErrorException.NotFound || ex.getCause() instanceof HttpClientErrorException.NotFound)) {