Fix oversized subscription requests with slicing

[Caideyipi]

Description

Subscription requests could exceed the negotiated Thrift frame size. This change:

• Slices oversized subscription request bodies using the smaller client/server frame limit, with 1 KiB reserved for RPC and slice metadata. The receiver validates ordering, metadata and total length before reassembling, with a bounded reassembly size.
• Batches heartbeat and commit contexts by serialized size, then merges the responses. Commit batches reserve headroom for echoed contexts and progress in the response.
• Keeps handshake and close unsliced because they control the connection lifecycle; an oversized one fails locally instead of sending an invalid frame.
• Leaves requests that fit in one frame unchanged. A server must support the new SLICE request type before a client can send oversized requests to it.

Verification

• Full reactor mvn test-compile -DskipTests, including -P with-zh-locale.
• Targeted PipeSubscribeSliceReqTest, SubscriptionProviderSlicingTest, and SubscriptionReceiverV1Test: 26 tests passed.
• Spotless, Checkstyle and git diff --check passed.

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This PR has:

• been self-reviewed.
• added comments explaining non-obvious choices.
• added unit tests for the new paths.

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Key changed/added classes (or packages if there are too many classes) in this PR

PipeSubscribeSliceReq, PipeSubscribeSliceReqBuilder, PipeSubscribeSliceReqHandler, AbstractSubscriptionProvider, SubscriptionReceiverV1.