fix(sse): preserve event state and validate streaming responses

[rupayon123]

Problem and change

Bring SSE parsing and streaming behavior in line with event-stream semantics. Preserve empty data fields and persistent event IDs/retry intervals; accept CR, LF, CRLF and a leading UTF-8 BOM; and reject invalid retry values.

The streaming follow-up fixes effective event filtering: untyped events use message, and event names match exactly rather than case-insensitively. Validate the response media type before creating an output log, so an HTML/JSON error response cannot overwrite a prior log. Support data lines smaller than 1 MiB with a documented bounded scanner rather than failing on ordinary 128 KiB payloads.

Custom SSE headers now honor case-insensitive Host overrides through the request host field, and malformed headers are rejected before making a network request.

Validation

• Parser regressions cover event state, retry fields, delimiters/BOM and 128 KiB/oversized lines.
• HTTP-server regressions cover filtered/default/case-distinct event types and valid/invalid/missing Content-Type, including preservation of an existing log on rejection.
• The three September 11 regression groups failed before their fixes. Full env -u NO_COLOR go test ./..., go vet ./..., formatting and diff checks pass after each change.
• New HTTP-server regressions reproduce the ignored Host override and malformed-header request before the fix. Full Go tests, vet, build, formatting and diff checks pass after the header change. Remote CI for this new head is checked separately; the earlier passing run is not evidence for the new commit.
• golangci-lint is unavailable locally. No claim of automatic reconnection is made.

Prepared with AI assistance.

[rupayon123]

Additional fixes pushed to this branch:

• d750be0: Stop SSE consumption on output write failure.

Full Go tests, go vet and build pass, including a live local streaming server that previously hung after an output failure. The stream now closes promptly and preserves the original write error. A subsequent cleanup separates event formatting from connection lifecycle. Today's changes have no new findings under golangci-lint v2.13.2 using a migrated copy of the repository configuration. This is a scoped lint result: a full CLI-branch scan still reports 93 findings outside today's changes; the legacy v1 linter cannot read the installed Go 1.27 export format.

Prepared with AI assistance. Changes are submitted for review; this is not a claim of maintainer approval.