fix: softFail noisy 'Not Acceptable' Accept-header errors from SDK

[MQ37]

Context

WebStandardStreamableHTTPServerTransport throws Not Acceptable: Client must accept ... from both handlePostRequest and handleGetRequest when the client's Accept header is missing one of the required MIME types (application/json, text/event-stream). The SDK already responds HTTP 406 to the caller, then forwards the error to onerror — where we currently log it as [MCP Error] and trip Mezmo alerts. Same class of client-misconfiguration noise we already soft-fail via #787 / #795.

Solution

Add the SDK error prefix Not Acceptable: Client must accept to the existing soft-fail filter in setupErrorHandling(). The prefix covers both variants the SDK emits (GET-path "must accept text/event-stream" and POST-path "must accept both application/json and text/event-stream").

Worth your attention

• Refactored the regex into an Array.join('|') builder — the previous one-liner was already brushing the 160-char max-len cap; a 5th alternation would break it. Each pattern now lives on its own line aligned with the comment block above.
• No tests added — follows the precedent from fix: Handle SSE controller closed state in error logging #795 ("Drop the SSE softFail unit test"), the filter is a string-match guard with no behavioral branching worth a unit test.