test(legacy): cover the v0.2 decoder ABI

The v0.2 decoder implementation and C boundary were already Rust-owned, but
its public error predicate, frame-size sentinel behavior, and opaque streaming
context lifecycle lacked focused Rust coverage. Add tests for malformed input,
content-size error reporting, one-shot decoding, reset behavior, and idempotent
context release so future legacy changes are checked at the ABI boundary.

Test Plan:
- Focused v0.2 Cargo tests under both feature sets -- 6 passed each
- `make -C tests -j2 test-legacy` -- passed
- `cargo +nightly fmt --manifest-path rust/Cargo.toml -- --check` -- passed
- Scoped `git diff --check` -- passed

rust/src/legacy/zstd_v02.rs

@@ -1710,6 +1710,96 @@ mod tests {
                 decode(&RAW_FRAME[..RAW_FRAME.len() - 1], 32).unwrap_err(),
                 ERROR(ZstdErrorCode::SrcSizeWrong)
             );
+            assert_eq!(
+                decode(RAW_FRAME, 4).unwrap_err(),
+                ERROR(ZstdErrorCode::DstSizeTooSmall)
+            );
+        }
+    }
+
+    #[test]
+    fn public_error_predicate_matches_the_c_abi() {
+        assert_eq!(ZSTDv02_isError(0), 0);
+        assert_eq!(ZSTDv02_isError(ERROR(ZstdErrorCode::Generic)), 1);
+        assert_eq!(ZSTDv02_isError(ERROR(ZstdErrorCode::SrcSizeWrong)), 1);
+        assert_eq!(ZSTDv02_isError(ERROR(ZstdErrorCode::MaxCode)), 0);
+        assert_eq!(ZSTDv02_isError(usize::MAX), 1);
+    }
+
+    #[test]
+    fn frame_size_abi_writes_error_and_content_size_sentinel() {
+        unsafe {
+            let mut c_size = 0xA5A5_A5A5usize;
+            let mut d_bound = 0x5A5A_5A5A_5A5A_5A5Au64;
+            ZSTDv02_findFrameSizeInfoLegacy(
+                RAW_FRAME.as_ptr() as *const c_void,
+                RAW_FRAME.len() - 1,
+                &mut c_size,
+                &mut d_bound,
+            );
+            assert_eq!(c_size, ERROR(ZstdErrorCode::SrcSizeWrong));
+            assert_eq!(d_bound, ZSTD_CONTENTSIZE_ERROR);
+
+            let mut bad_magic = RAW_FRAME.to_vec();
+            bad_magic[0] ^= 1;
+            ZSTDv02_findFrameSizeInfoLegacy(
+                bad_magic.as_ptr() as *const c_void,
+                bad_magic.len(),
+                &mut c_size,
+                &mut d_bound,
+            );
+            assert_eq!(c_size, ERROR(ZstdErrorCode::PrefixUnknown));
+            assert_eq!(d_bound, ZSTD_CONTENTSIZE_ERROR);
+        }
+    }
+
+    #[test]
+    fn context_abi_preserves_stream_state_and_supports_one_shot_decode() {
+        unsafe {
+            let dctx = ZSTDv02_createDCtx();
+            assert!(!dctx.is_null());
+            assert_eq!(ZSTDv02_nextSrcSizeToDecompress(dctx), 4);
+
+            let mut output = [0u8; 32];
+            assert_eq!(
+                ZSTDv02_decompressContinue(
+                    dctx,
+                    output.as_mut_ptr() as *mut c_void,
+                    output.len(),
+                    RAW_FRAME.as_ptr() as *const c_void,
+                    3,
+                ),
+                ERROR(ZstdErrorCode::SrcSizeWrong)
+            );
+            assert_eq!(ZSTDv02_nextSrcSizeToDecompress(dctx), 4);
+
+            let mut bad_magic = RAW_FRAME.to_vec();
+            bad_magic[0] ^= 1;
+            assert_eq!(
+                ZSTDv02_decompressContinue(
+                    dctx,
+                    output.as_mut_ptr() as *mut c_void,
+                    output.len(),
+                    bad_magic.as_ptr() as *const c_void,
+                    4,
+                ),
+                ERROR(ZstdErrorCode::PrefixUnknown)
+            );
+            assert_eq!(ZSTDv02_nextSrcSizeToDecompress(dctx), 4);
+
+            let decoded = ZSTDv02_decompressDCtx(
+                dctx as *mut c_void,
+                output.as_mut_ptr() as *mut c_void,
+                output.len(),
+                RAW_FRAME.as_ptr() as *const c_void,
+                RAW_FRAME.len(),
+            );
+            assert_eq!(decoded, b"raw v0.2!!!".len());
+            assert_eq!(&output[..decoded], b"raw v0.2!!!");
+            assert_eq!(ZSTDv02_resetDCtx(dctx), 0);
+            assert_eq!(ZSTDv02_nextSrcSizeToDecompress(dctx), 4);
+            assert_eq!(ZSTDv02_freeDCtx(dctx), 0);
+            assert_eq!(ZSTDv02_freeDCtx(ptr::null_mut()), 0);
         }
     }