…ng one chunk

streamableHttp.test.ts asserted on multiple SSE events after a single
reader.read() call, assuming Node's fetch would coalesce them into one
chunk. That assumption doesn't hold on newer Node versions, where
closely-timed events can arrive as separate chunks across separate
reads, failing the test before the later event is read.

Adds a readUntilContains helper that accumulates decoded text across
reads until the expected content appears (or times out), and uses it
in the two tests that were asserting on multiple SSE events from a
single read.

…+ migration guide

On SDK v1, server.tool() expects a raw shape ({ name: z.string() })
rather than a ZodObject, but nothing about the two failure modes
points a reader at the actual mistake:

- v1 <=1.21: tools/list crashes with
  "Cannot read properties of null (reading '_def')"
- v1 1.22-1.26: registration succeeds but publishes an empty
  {"type":"object"} schema, so every client silently strips the
  tool's arguments

v1 >=1.28 already throws a clear error, but the older ranges are
still widely deployed and easy to hit when copying v2-style
examples (which use z.object()) into v1 code. Adds a troubleshooting
entry keyed on both observable symptoms, and a short callout in the
v2 migration guide next to the registerTool example, so both search
paths land on the explanation.

Reported with full repro matrix in modelcontextprotocol#2627.