feat(raptor): add Psi tree builder with original-space ranking and safe migration (#14679)

### What problem does this PR solve?

Closes #14674.

This PR improves RAPTOR configuration and tree construction while
preserving the existing RAPTOR behavior as the default.

RAPTOR currently builds summary layers with the original UMAP + GMM
clustering path. This PR keeps that default path, and adds:

- A hidden backend tree-builder option:
  - `tree_builder="raptor"`: default, existing RAPTOR behavior.
- `tree_builder="psi"`: rank-aware Psi-style tree builder using original
embedding-space cosine ranking.
- A user-facing clustering method option for the default RAPTOR builder:
  - `clustering_method="gmm"`: existing default.
- `clustering_method="ahc"`: agglomerative hierarchical clustering path.
- A RAPTOR UI setting for `Clustering method` and `Max cluster`.

### What changed

#### Backend

- Added `tree_builder` support for RAPTOR/Psi.
- Added `clustering_method` support for GMM/AHC.
- Kept existing RAPTOR + GMM as the default.
- Added Psi tree building from original-space cosine similarity.
- Added bucketed Psi building controls for large inputs:
  - `raptor.ext.psi_exact_max_leaves`
  - `raptor.ext.psi_bucket_size`
- Added method-aware RAPTOR summary metadata using existing
`extra.raptor_method`.
- Avoided adding a dedicated DB schema field for experimental method
tracking.
- Added cleanup/migration logic to avoid mixing stale RAPTOR summary
trees.
- Added defensive checks for Psi tree construction and summary failures.

#### Frontend/UI

- Added `Clustering method` in RAPTOR settings with `GMM` and `AHC`.
- Added/kept `Max cluster` in RAPTOR settings.
- Enlarged max cluster UI limit to `1024`, matching backend validation.
- Kept AHC editable even when a RAPTOR task has already finished.
- Fixed the UI save payload so `clustering_method` and `tree_builder`
are serialized through `parser_config.raptor.ext`, avoiding backend
validation errors for extra top-level RAPTOR fields.

Example saved RAPTOR config:

```json
{
  "raptor": {
    "max_cluster": 317,
    "ext": {
      "clustering_method": "ahc",
      "tree_builder": "raptor"
    }
  }
}

Co-authored-by: CaptainTimon <CaptainTimon@users.noreply.github.com>

test/testcases/test_http_api/test_dataset_management/test_update_dataset.py

@@ -583,6 +583,10 @@ class TestDatasetUpdate:
             {"raptor": {"max_cluster": 512}},
             {"raptor": {"max_cluster": 1024}},
             {"raptor": {"random_seed": 0}},
+            {"raptor": {"clustering_method": "gmm"}},
+            {"raptor": {"clustering_method": "ahc"}},
+            {"raptor": {"tree_builder": "raptor"}},
+            {"raptor": {"tree_builder": "psi"}},
         ],
         ids=[
             "auto_keywords_min",
@@ -633,6 +637,10 @@ class TestDatasetUpdate:
             "raptor_max_cluster_mid",
             "raptor_max_cluster_max",
             "raptor_random_seed_min",
+            "raptor_clustering_method_gmm",
+            "raptor_clustering_method_ahc",
+            "raptor_tree_builder_raptor",
+            "raptor_tree_builder_psi",
         ],
     )
     def test_parser_config(self, HttpApiAuth, add_dataset_func, parser_config):
@@ -707,6 +715,10 @@ class TestDatasetUpdate:
             ({"raptor": {"random_seed": -1}}, "Input should be greater than or equal to 0"),
             ({"raptor": {"random_seed": 3.14}}, "Input should be a valid integer"),
             ({"raptor": {"random_seed": "string"}}, "Input should be a valid integer"),
+            ({"raptor": {"clustering_method": "unknown"}}, "Input should be 'gmm' or 'ahc'"),
+            ({"raptor": {"clustering_method": None}}, "Input should be 'gmm' or 'ahc'"),
+            ({"raptor": {"tree_builder": "ahc"}}, "Input should be 'raptor' or 'psi'"),
+            ({"raptor": {"tree_builder": None}}, "Input should be 'raptor' or 'psi'"),
             ({"delimiter": "a" * 65536}, "Parser config exceeds size limit (max 65,535 characters)"),
         ],
         ids=[
@@ -763,6 +775,10 @@ class TestDatasetUpdate:
             "raptor_random_seed_min_limit",
             "raptor_random_seed_float_not_allowed",
             "raptor_random_seed_type_invalid",
+            "raptor_clustering_method_invalid",
+            "raptor_clustering_method_none_invalid",
+            "raptor_tree_builder_invalid",
+            "raptor_tree_builder_none_invalid",
             "parser_config_type_invalid",
         ],
     )