Fix panic in JSONB decoder on invalid version byte

[jrey8343]

Fixes #4157

Summary

Replaces assert_eq! with proper error handling in the JSONB decoder to prevent panics on untrusted database input.

Changes

• File: sqlx-postgres/src/types/json.rs
• Change: Replace assertion with conditional check + error return
• Lines changed: 8 insertions, 5 deletions

Why This Fix Is Needed

The Decode trait contract requires implementations to return Result<T, Error>, but the current code uses assert_eq! which panics on invalid input. This violates the trait contract and prevents applications from handling errors gracefully.

Before

assert_eq!(
    buf[0], 1,
    "unsupported JSONB format version {}; please open an issue",
    buf[0]
);

After

// Check JSONB version byte - PostgreSQL currently only supports version 1
if buf[0] != 1 {
    return Err(format!(
        "unsupported JSONB format version {} (expected 1)",
        buf[0]
    )
    .into());
}

Testing

• ✅ Compiles successfully (cargo check -p sqlx-postgres)
• ✅ Maintains identical behavior for valid input (version byte = 1)
• ✅ Returns proper error for invalid input instead of panicking
• ✅ Discovered and validated through fuzzing

Impact

This fix prevents denial-of-service scenarios where malformed JSONB data could crash applications. It allows applications to handle database errors gracefully through the standard Result error handling pattern.