fix(ssestream): synchronize decoder registry access

[tvost2]

Fixes #886

RegisterDecoder and NewDecoder accessed the package-level decoder registry without synchronization, allowing concurrent map writes and race detector failures. Protect registry reads and writes with an RWMutex and add concurrent coverage.

Validation:

• go test -race ./packages/ssestream
• go test ./packages/ssestream
• go test ./... (fails in existing tests because the localhost:4010 mock server is not running; also TestMultipartFilenameMetadata/backslash fails independently)