node: Consolidate borsch library usage

[djb15]

I spent some time looking through the existing tests and adding a couple of new ones to check whether we're hitting all the edge cases we care about. Not a place we want to be making a mistake, so I'm happy to abandon these changes if the risks outweigh the upside

[johnsaigle]

This is a change in behaviour, right? Previously I believe this returned an error, and that's probably safer

[johnsaigle]

If gagliardetto doesn't do this then I think that's arguably a bug in that library

[johnsaigle]

Got this from an LLM FWIW:

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_Borsh rejects trailing bytes. The reference implementation (borsh-rs) treats any unconsumed bytes after deserialization as an error.
The high-level entry points enforce this:

• from_slice / try_from_slice — after T::deserialize, check if !v_mut.is_empty() and return Err(InvalidData, "Not all bytes read") (borsh-rs/borsh/src/de/mod.rs).
• from_reader / try_from_reader — after deserializing, attempt to read 1 more byte; success (non-EOF) triggers the same "Not all bytes read" error.
  The low-level deserialize / deserialize_reader methods themselves do not check — they only consume what the type needs. So the "no trailing bytes" rule is enforced at the API boundary, not per-field. The written spec (the README pseudocode) doesn't call it out explicitly, but the reference implementation's behavior is the de facto standard, and the error constant ERROR_NOT_ALL_BYTES_READ makes the stance unambiguous: the entire input must be consumed._