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>

web/src/components/ui/radio.tsx

@@ -13,6 +13,7 @@ type RadioProps = {
   checked?: boolean;
   disabled?: boolean;
   onChange?: (checked: boolean) => void;
+  testId?: string;
   children?: React.ReactNode;
 } & Omit<
   React.InputHTMLAttributes<HTMLInputElement>,
@@ -25,6 +26,7 @@ function Radio({
   checked,
   disabled,
   onChange,
+  testId,
   children,
   ...props
 }: RadioProps) {
@@ -65,6 +67,7 @@ function Radio({
         onChange={handleChange}
         disabled={mergedDisabled}
         className={cn('peer absolute size-[1px] opacity-0', className)}
+        data-testid={testId}
         {...props}
         name={groupContext?.name}
       />
@@ -151,9 +154,11 @@ const Group = React.forwardRef<HTMLDivElement, RadioGroupProps>(
           )}
         >
           {React.Children.map(children, (child) => {
-            if (!React.isValidElement<RadioProps>(child)) return child;
+            if (!React.isValidElement<RadioProps>(child)) {
+              return child;
+            }
             return React.cloneElement(child, {
-              disabled: disabled || child.props?.disabled,
+              disabled: disabled || child.props.disabled,
             });
           })}
         </div>