fix(flexbuffers): harden Rust Reader against panics from untrusted input

[jrey8343]

Summary

The FlexBuffers Rust Reader has multiple code paths that panic when processing malformed or malicious input, enabling denial of service for any application that deserializes FlexBuffers from untrusted sources.

This patch systematically replaces all unchecked buffer accesses and unchecked integer arithmetic in the FlexBuffers reader with checked alternatives that return Err instead of panicking.

Bounds check failures (direct buffer[addr..] indexing → .get())

• get_bool() (mod.rs): sliced self.buffer[self.address..self.address + width] without bounds check
• get_key_len() (mod.rs): sliced self.buffer[self.address..] without bounds check
• read_usize() (mod.rs): sliced &buffer[address..] and used cursor[0] without bounds check
• MapReader::lazy_strcmp() (map.rs): sliced self.buffer[key_addr..] without bounds check (had a TODO: Can we know this won't OOB and panic? comment)

Integer overflow (address + length → checked_add/checked_mul)

• get_str(), get_blob(), get_key() (mod.rs): self.address + self.length() overflows when length is read from malicious input
• get_slice() (mod.rs): self.address + self.length() * size_of::<T>() — double overflow risk
• VectorReader::get_elem_type(), VectorReader::index() (vector.rs): address + length * width
• MapReader::index_key(), MapReader::usize_index() (map.rs): multiple address + width * count calculations

Crash inputs (found by fuzzing with cargo-fuzz/libFuzzer)

Input (hex)	Panic location	Root cause
5d 79 6b 02	mod.rs:326	get_bool() — range end index 8 out of range for slice of length 4
4f 71 02 6b 02	mod.rs:326	Same, via serde deserializer
00*14 17 02	mod.rs:378	get_str() — attempt to add with overflow

All three inputs now return Err(FlexbufferOutOfBounds) instead of panicking.

Test plan

• cargo check passes for flexbuffers crate
• All three crash inputs execute without panic (verified with cargo fuzz run <target> <crash> -- -runs=1)
• 30-second fuzz run: 5.7M iterations, 0 crashes on fuzz_flexbuffers_reader

Fixes #8923

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