Add Spector case for serializing fractional duration encoded as integer

[Copilot]

Existing encode/duration scenarios only used whole-number payloads, so they couldn't catch the bug from #10831 where a duration with a fractional sub-second component was serialized as a floating point number (e.g. 123.45) instead of an integer.

Changes (packages/http-specs/specs/encode/duration/)

• main.tsp: New int32-seconds-fractional scenarios in the Query, Property, and Header namespaces. Each constructs a 35.625-second duration encoded as int32 seconds:

  @route("/int32-seconds-fractional")
  @scenario
  @scenarioDoc("""
    ...The duration is 35.625 seconds, e.g. TimeSpan.FromSeconds(35.625) in C#.
    Even though the underlying value is fractional, the client must serialize it
    as an integer (not a floating point number), e.g. `input=36`.
    """)
  op int32SecondsFractional(
    @query @encode(DurationKnownEncoding.seconds, int32) input: duration,
  ): NoContentResponse;

• mockapi.ts: New handlers (createBodyIntServerTests, createQueryIntServerTests, createHeaderIntServerTests) assert the received value is an integer and reject floating point. Validation checks integer-ness rather than an exact value, since truncate-vs-round semantics across languages is still unresolved in the issue discussion — this verifies the core contract ("serializes as an int") without prescribing a rounding mode.

• spec-summary.md: Regenerated.

Open question

The cross-language semantic for converting a fractional duration to an integer (truncate vs. round) remains undecided per the issue thread. The mock is intentionally lenient so it passes under either choice while still failing on float output; tightening to an exact value can follow once the semantic is settled.