feat(serde): support standalone FileScanTask JSON in core

[kamcheungting-db]

Summary

Add standalone core JSON serialization and deserialization for FileScanTask.

Unlike REST scan responses, core task JSON is self-contained: it embeds the schema, partition spec, data file, delete files, range, and residual filter.

Example

{
  "task-type": "file-scan-task",
  "schema": {"type": "struct", "schema-id": 0, "fields": []},
  "spec": {"spec-id": 0, "fields": []},
  "data-file": {
    "spec-id": 0,
    "content": "data",
    "file-path": "/data.parquet",
    "file-format": "parquet",
    "partition": [],
    "record-count": 10,
    "file-size-in-bytes": 100
  },
  "start": 0,
  "length": 100,
  "delete-files": [],
  "residual-filter": true
}

Core versus REST delete files

Core JSON inlines delete files:

"delete-files": [{"content": "position-deletes", "file-path": "/delete.parquet"}]

REST may instead refer to a sibling response array:

"delete-file-references": [0]

Those references only make sense while parsing the complete REST response, so core serde rejects them and leaves them in the REST-specific path.

Split handling

C++ does not currently support split FileScanTasks. This PR accepts only the whole file:

"start": 0,
"length": 100

For a 100-byte file, values such as "start": 10 or "length": 90 are rejected instead of silently becoming a whole-file task. The REST parser now performs the same check.

The implementation also validates data/delete file roles, null delete entries, task type, and required fields. A missing residual filter defaults to alwaysTrue.

Stack

This is 3 of 4 and depends on #975:

1. Move DataFile serde into core (refactor(serde): move DataFile JSON handling from REST to core #974)
2. Java-compatible DataFile JSON (fix(serde): make DataFile JSON compatible with Java Iceberg #975)
3. This PR: core FileScanTask JSON serde
4. Strict schema and partition ID parsing (fix(serde): reject invalid schema and partition IDs in JSON #940)

Each PR targets main; later PRs contain the commits from earlier PRs.

Testing

• 21 core FileScanTask tests passed.
• Focused core and REST tests: 51 tests run, 50 passed, 1 existing row-lineage test skipped.
• Full stack: 142 JSON serde tests passed.
• Full stack: 332 REST tests passed; 1 existing test skipped.

[kamcheungting-db]

I split the original change into a reviewable stack:

1. refactor(serde): move DataFile JSON handling from REST to core #974 — mechanical DataFile serde move into core
2. fix(serde): make DataFile JSON compatible with Java Iceberg #975 — Java-compatible DataFile JSON
3. feat(serde): support standalone FileScanTask JSON in core #939 — core FileScanTask serde and split validation
4. fix(serde): reject invalid schema and partition IDs in JSON #940 — strict schema and partition ID parsing

For the incremental diff that belongs only to this PR, use:
kamcheungting-db/iceberg-cpp@datafile-serde-java...filescan-task-serde

The key distinction in this layer is:

• Core task JSON is self-contained and uses inline "delete-files": [...].
• REST may use response-local "delete-file-references": [0], which remains REST-only.
• For a 100-byte file, only start = 0 and length = 100 are accepted; a split such as start = 10 is rejected instead of silently becoming a whole-file task.

The PR description now includes a complete JSON example and the stack order.