HoodieIndexUtils.filterKeysFromFile hardcodes AVRO fallback, ignoring configured record merger type

### Describe the problem

In [`HoodieIndexUtils.filterKeysFromFile`](https://github.com/apache/hudi/blob/master/hudi-client/hudi-client-common/src/main/java/org/apache/hudi/index/HoodieIndexUtils.java#L253) the record type used to open the base file reader for key filtering is computed as:

```java
HoodieRecord.HoodieRecordType recordType =
HoodieFileFormat.fromFileExtension(FSUtils.getFileExtension(filePath.toString()))
.resolveRecordType(HoodieRecord.HoodieRecordType.AVRO);
```

`resolveRecordType` only overrides the fallback when the file format **requires** a specific record type (currently only Lance). For all other formats (Parquet, ORC, HFile) the AVRO fallback is hardcoded and ignores the merger's configured record type. This predates the Lance work (#18375).

### Impact

If a user configures a SPARK-type merger for a Parquet table, key filtering during index lookup will still open an AVRO reader, incurring unnecessary Avro<->InternalRow conversions in the key-filter code path.

This is not a correctness bug — `filterRowKeys` only reads record keys — but it is an efficiency/consistency gap flagged during review of #18375 (cf. [review thread](https://github.com/apache/hudi/pull/18375#discussion_r3036297458)).

### Proposed fix

Thread the configured merger's record type into `filterKeysFromFile` (or read it from config via an argument) and pass it as the fallback to `resolveRecordType` instead of hardcoding `AVRO`.

### Context

- Raised by @rahil-c in https://github.com/apache/hudi/pull/18375#discussion_r3069697589
- File: `hudi-client/hudi-client-common/src/main/java/org/apache/hudi/index/HoodieIndexUtils.java` around line 254 (the `// TODO: AVRO fallback` comment).

Start in hudi-client/hudi-client-common/src/main/java/org/apache/hudi/index/HoodieIndexUtils.java at filterKeysFromFile and inspect its callers and the review context from #18375. Trace how the configured merger record type is available, then verify that it is used as the fallback while formats requiring a specific type, such as Lance, still override it. Done means key filtering no longer hardcodes AVRO for configurable formats.

Written by the indexing model from the issue text.