feat: add Perplexity contextualized embeddings API as a new model provider (#13709)

### What problem does this PR solve?

Adds Perplexity contextualized embeddings API as a new model provider,
as requested in #13610.

- `PerplexityEmbed` provider in `rag/llm/embedding_model.py` supporting
both standard (`/v1/embeddings`) and contextualized
(`/v1/contextualizedembeddings`) endpoints
- All 4 Perplexity embedding models registered in
`conf/llm_factories.json`: `pplx-embed-v1-0.6b`, `pplx-embed-v1-4b`,
`pplx-embed-context-v1-0.6b`, `pplx-embed-context-v1-4b`
- Frontend entries (enum, icon mapping, API key URL) in
`web/src/constants/llm.ts`
- Updated `docs/guides/models/supported_models.mdx`
- 22 unit tests in `test/unit_test/rag/llm/test_perplexity_embed.py`

Perplexity's API returns `base64_int8` encoded embeddings (not
OpenAI-compatible), so this uses a custom `requests`-based
implementation. Contextualized vs standard model is auto-detected from
the model name.

Closes #13610

### Type of change

- [x] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update

conf/llm_factories.json

@@ -6317,6 +6317,38 @@
             "status": "1",
             "rank": "100",
             "llm": []
+        },
+        {
+            "name": "Perplexity",
+            "logo": "",
+            "tags": "TEXT EMBEDDING",
+            "status": "1",
+            "llm": [
+                {
+                    "llm_name": "pplx-embed-v1-0.6b",
+                    "tags": "TEXT EMBEDDING,32000",
+                    "max_tokens": 32000,
+                    "model_type": "embedding"
+                },
+                {
+                    "llm_name": "pplx-embed-v1-4b",
+                    "tags": "TEXT EMBEDDING,32000",
+                    "max_tokens": 32000,
+                    "model_type": "embedding"
+                },
+                {
+                    "llm_name": "pplx-embed-context-v1-0.6b",
+                    "tags": "TEXT EMBEDDING,32000",
+                    "max_tokens": 32000,
+                    "model_type": "embedding"
+                },
+                {
+                    "llm_name": "pplx-embed-context-v1-4b",
+                    "tags": "TEXT EMBEDDING,32000",
+                    "max_tokens": 32000,
+                    "model_type": "embedding"
+                }
+            ]
         }
     ]
 }