mtmd, server: add "placeholder bitmap" for counting tokens , add */input_tokens API (#23913)

* mtmd: add "placeholder bitmap" for counting tokens w/o preprocessing

* fast path skip preproc for placeholder

* fix build

* correct the api

* add server endpoint + tests

* add object name

* update docs

* add proxy handling

* fix build

* fix audio input path

* use is_placeholder in process_mtmd_prompt()

* nits

* nits (2)

* docs: clarify chat/completions/input_tokens is not official

* fix merge problem

tools/server/README.md

@@ -1447,6 +1447,36 @@ See [OpenAI Embeddings API documentation](https://platform.openai.com/docs/api-r
   }'
   ```
 
+### POST `/v1/responses/input_tokens`: Token Counting
+
+Similar to [Response input token counts API](https://developers.openai.com/api/reference/python/resources/responses/subresources/input_tokens/methods/count).
+
+Example response:
+
+```json
+{
+  "object": "response.input_tokens",
+  "input_tokens": 11
+}
+```
+
+### POST `/v1/chat/completions/input_tokens`: Token Counting
+
+Similar to [Response input token counts API](https://developers.openai.com/api/reference/python/resources/responses/subresources/input_tokens/methods/count), but accepts a chat completion body as input.
+
+Note: This is not an official OAI endpoint, but is added for completeness and convenience.
+
+Example response:
+
+```json
+{
+  "object": "response.input_tokens",
+  "input_tokens": 11
+}
+```
+
+## Anthropic-compatible API Endpoints
+
 ### POST `/v1/messages`: Anthropic-compatible Messages API
 
 Given a list of `messages`, returns the assistant's response. Streaming is supported via Server-Sent Events. While no strong claims of compatibility with the Anthropic API spec are made, in our experience it suffices to support many apps.