fix(bedrock): handle EOF consistently, fix stale events, and surface exceptions

[rishabh-emergent]

Summary

Three fixes for the Bedrock EventStream decoder in bedrock/bedrock.go.

Bug 1: EOF exposed as error (fixes #110)

The SSE decoder (eventStreamDecoder) returns nil from scn.Err() on normal stream end. The EventStream decoder (eventstreamDecoder) exposes io.EOF as an error via Err(). This forces consumers to special-case Bedrock EOF handling.

// Before: EOF stored as error
e.err = err
return false

// After: EOF filtered, matching SSE decoder behavior
if err == io.EOF {
    return false
}
e.err = err
return false

Bug 2: Stale event on non-chunk event types

When eventType != "chunk", e.evt is not updated but Next() returns true. Consumers re-process the previous event's data.

// Before: falls through to return true with stale e.evt
if eventType.String() == "chunk" {
    // ... update e.evt
}
// return true ← stale event!

// After: skip non-chunk events
if eventType.String() == "chunk" {
    // ... update e.evt
    return true
}
return e.Next() // skip, try next event

Bug 3: ExceptionMessageType silently dropped (fixes #71)

Go case statements don't fall through. ExceptionMessageType had its own handler that never reached ErrorMessageType, causing Bedrock exceptions to be silently dropped.

// Before: two separate cases, exception handler doesn't fall through
case eventstreamapi.ExceptionMessageType:
    // ... sets e.err, returns false
case eventstreamapi.ErrorMessageType:
    // ... never reached for exceptions

// After: merged into single case
case eventstreamapi.ExceptionMessageType, eventstreamapi.ErrorMessageType:
    // handles both uniformly

Impact

These bugs cause silent data loss when streaming from Bedrock:

• EOF masking prevents consumers from detecting stream errors (Bedrock streaming decoder error not returned #110)
• Stale events cause duplicate processing of previous event data
• Dropped exceptions cause empty response streams for invalid requests (Error handling code for Bedrock doesn't work for exception message types #71)

We hit all three in production (391 errors/day) when Bedrock streams were prematurely terminated — the EOF was silently swallowed, and the incomplete response caused downstream failures.

Tests

All existing tests pass: go test ./bedrock/... -v