Stability improvements for compressed BOCs and external messages

[bvscd]

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[Copilot]

Pull request overview

This PR improves stability around compressed BOC handling, RPC request sizing, and external-message pool lifecycle behavior in the node/collator path.

Changes:

• Adds BOC decompression/compression safeguards for empty input, special-cell data length, and improved-structure node/root count caps.
• Changes external-message completion to operate on raw message IDs, postponing accepted messages and deleting rejected messages without norm-hash sibling cleanup.
• Adds JSON-RPC/REST body size limits and updates related tests for postponed-message generation timing and BOC node-count limits.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/block/src/boc_compression.rs	Adds improved BOC node/root count caps and safer top-level decompression handling.
src/block/src/cell/builder.rs	Adds length checks before Merkle proof/update hash slicing.
src/block/src/tests/test_boc_compression.rs	Adds regression coverage for improved BOC node-count limits.
src/node/src/ext_messages.rs	Updates external-message admission, completion, postponement, cleanup, and iterator behavior.
src/node/src/rpc_server/mod.rs	Adds request body size limits and payload-too-large rejection handling.
src/node/src/validator/collator.rs	Routes accepted external messages to delay and rejected ones to raw-ID deletion.
src/node/src/engine_traits.rs	Updates external-message completion trait signature to use ID slices.
src/node/src/engine_operations.rs	Adapts engine implementation to the new completion signature.
src/node/bin/hardfork.rs	Adapts mock engine implementation to the new completion signature.
src/node/src/tests/test_helper.rs	Adapts test engine implementation to the new completion signature.
src/node/src/validator/tests/test_collator.rs	Adapts collator test engine implementation to the new completion signature.
src/node/src/collator_test_bundle.rs	Adapts collator test bundle implementation to the new completion signature.
src/node/src/tests/test_ext_messages.rs	Updates postponement timing expectations and completion calls.

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