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

web/src/constants/llm.ts

@@ -65,6 +65,7 @@ export enum LLMFactory {
   N1n = 'n1n',
   Avian = 'Avian',
   RAGcon = 'RAGcon',
+  Perplexity = 'Perplexity',
 }
 
 // Please lowercase the file name
@@ -135,6 +136,7 @@ export const IconMap = {
   [LLMFactory.N1n]: 'n1n',
   [LLMFactory.Avian]: 'avian',
   [LLMFactory.RAGcon]: 'ragcon',
+  [LLMFactory.Perplexity]: 'perplexity',
 };
 
 export const APIMapUrl = {
@@ -188,4 +190,6 @@ export const APIMapUrl = {
   [LLMFactory.PaddleOCR]: 'https://www.paddleocr.ai/latest/',
   [LLMFactory.N1n]: 'https://docs.n1n.ai',
   [LLMFactory.Avian]: 'https://avian.io',
+  [LLMFactory.Perplexity]:
+    'https://docs.perplexity.ai/docs/embeddings/quickstart',
 };