Is it possible to use polyline6 as input to matrix API, and if so, how?

I was trying to update code I use for both OSRM and Valhalla, and I noticed that my Valhalla all-to-all function does not use polyline encoding of the input.

According to this page in the OSRM docs: https://project-osrm.org/docs/v5.24.0/api/#general-options, one can specify the coordinates of a table query using polyline6.

There are lots of vague words in the Valhalla docs that imply that one can do the same thing here, but nothing concrete.

So I just tried it and it doesn't seem to work, although in the past I've been tripped up by issues with polyline encoding and JSON.

Anyway, my attempt looks like this:

```
sources_to_targets?json={"sources": "uk%7BviA%60%7DeadF%7B%7CiDfel%40%60tdL~hRyaxI_grEyaAhyKhxpItx~Eo%7BuGqisA", "targets": "uk%7BviA%60%7DeadF%7B%7CiDfel%40%60tdL~hRyaxI_grEyaAhyKhxpItx~Eo%7BuGqisA", "costing": "auto", "shape_format": "polyline6"}
```

(If I swap out the polyline stuff with actual coordinate pairs the usual way, it works fine.)

I was hoping that using the "shape-format" parameter would just work.

My server responds with:
```
{"error_code":100,"error":"Failed to parse json request","status_code":400,"status":"Bad Request"}
```
And the log messages on the server say:
```
2025/08/08 19:45:36.941261 [INFO] Got Loki Request 69
2025/08/08 19:45:39.200910 [INFO] Got Thor Request 69
2025/08/08 19:45:39.201083 [INFO] matrix::costmatrix
69 2025/08/08 19:45:39.457319 200 10903
70 2025/08/08 20:22:39.147051 GET /sources_to_targets?json=%7B%22sources%22:%20%22svvyiAfrp%7CcFdoOy_a%40%60%60fIb%7DbG%7DxtKqyQf_s%40g_eEdkbCh%7BvEl__Aj%7B%60%40%22,%20%22targets%22:%20%22svvyiAfrp%7CcFdoOy_a%40%60%60fIb%7DbG%7DxtKqyQf_s%40g_eEdkbCh%7BvEl__Aj%7B%60%40%22,%20%22costing%22:%20%22auto%22,%20%22shape_format%22:%20%22polyline6%22%7D HTTP/1.1
2025/08/08 20:22:39.147409 [INFO] Got Loki Request 70
2025/08/08 20:22:39.147687 [WARN] 400::Insufficiently specified required parameter 'locations' or 'sources & targets' request_id=70
70 2025/08/08 20:22:39.147922 400 275
```

A good request with just plain lat, lon pairs gives this on the server:

```
73 2025/08/08 20:35:56.993322 GET /sources_to_targets?json=%7B%22sources%22:%20%5B%7B%22lat%22:%2039.077773,%20%22lon%22:%20-120.156923%7D,%20%7B%22lat%22:%2039.239968,%20%22lon%22:%20-120.052065%7D,%20%7B%22lat%22:%2039.234203,%20%22lon%22:%20-120.016998%7D,%20%7B%22lat%22:%2039.276152,%20%22lon%22:%20-120.121543%7D,%20%7B%22lat%22:%2039.244327,%20%22lon%22:%20-120.050522%7D,%20%7B%22lat%22:%2039.22675,%20%22lon%22:%20-120.08242%7D,%20%7B%22lat%22:%2039.240383,%20%22lon%22:%20-120.028053%7D%5D,%20%22targets%22:%20%5B%7B%22lat%22:%2039.077773,%20%22lon%22:%20-120.156923%7D,%20%7B%22lat%22:%2039.239968,%20%22lon%22:%20-120.052065%7D,%20%7B%22lat%22:%2039.234203,%20%22lon%22:%20-120.016998%7D,%20%7B%22lat%22:%2039.276152,%20%22lon%22:%20-120.121543%7D,%20%7B%22lat%22:%2039.244327,%20%22lon%22:%20-120.050522%7D,%20%7B%22lat%22:%2039.22675,%20%22lon%22:%20-120.08242%7D,%20%7B%22lat%22:%2039.240383,%20%22lon%22:%20-120.028053%7D%5D,%20%22costing%22:%20%22auto%22%7D HTTP/1.1
2025/08/08 20:35:56.993647 [INFO] Got Loki Request 73
2025/08/08 20:35:57.857900 [INFO] Got Thor Request 73
2025/08/08 20:35:57.858124 [INFO] matrix::costmatrix
73 2025/08/08 20:35:58.022537 200 8970
```

so I know the server is working fine and I can query it fine, I just don't know how to query it with polyline encoded input.