docs(sandboxes): update Mistral Vibe guide for kits v3 (#26345)

## Description

The Mistral guide uses a v2 `spec.yaml` and injects credentials only for
`api.mistral.ai`. Update it to a v3 workload descriptor and companion
Dockerfile, with a pinned Vibe 2.25.8 installation and credential
injection for the inference, account, and managed-config hosts.

Keep the build-and-run walkthrough and add a FastAPI repair exercise
using the published community kit. Include three screenshots showing the
credential placeholder, Vibe's repair summary, and the model menu.
Readers verify the repair with pytest inside the sandbox and a Git diff
from the host repository.

## Validation

- Formatted and linted the changed guide with repository-pinned rumdl
and `scripts/lint.sh`: zero errors, warnings, or suggestions
- Validated the embedded descriptor against the upstream v3 JSON schema
and checked the three literal network hosts and authorization mappings
- Checked front matter, internal links, screenshot paths, and the public
sample configuration
- Ran fresh Gemini reader and technical reviews through Pi in SBX, with
tools, skills, and prior context disabled; fixed the findings and
received final READABLE and PASS verdicts
- Screenshots are from the interactive Vibe 2.25.8 sample; the earlier
sample run used sbx 0.46.0-rc5

Full Docker-based site validation and a rebuild of the tutorial kit
remain pending: the host Docker engine was unavailable. Schema and prose
checks do not establish image-build or runtime success.

## Reviews

- [x] Technical review through Gemini, plus descriptor schema validation
- [x] Editorial review through Gemini and local prose/lint checks
- [ ] Product review

---------

Signed-off-by: Oleg Šelajev <shelajev@gmail.com>
Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>
Co-authored-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

Author: shelajev