ci: use pinned versions in cedarling test workflow

> _⚠️ Potential issue_ | _🟡 Minor_
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> 🧩 Analysis chain
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> 🏁 Script executed:
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> ```shell
> #!/bin/bash
> # Description: Show Rust toolchain pins and explicit cargo toolchain overrides.
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> echo "Repo Rust toolchain:"
> sed -n '1,20p' jans-cedarling/rust-toolchain.toml
>
> echo
> echo "Workflow Rust toolchain declarations and overrides:"
> rg -n -C2 'toolchain:\s*(stable|"1\.95\.0"|'\''1\.95\.0'\'')|cargo\s+\+(stable|1\.95\.0)|rustup component add --toolchain' .github/workflows/test-cedarling.yml
> ```
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> Repository: JanssenProject/jans
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> Length of output: 2607
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> ---
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> **Keep the Rust toolchain pin consistent across Cedarling CI.**
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> The python_tests job correctly uses `1.95.0` (matching `jans-cedarling/rust-toolchain.toml`), but other jobs still install `stable`. Once stable advances, CI will validate different compilers depending on the job, creating inconsistent test coverage.
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> Update all Cedarling workflow jobs to use the pinned `1.95.0` version:
> - Change `toolchain: stable` to `toolchain: "1.95.0"` in job `with:` blocks (lines 26, 59, 108, 138, 213, 250)
> - Change `cargo +stable` to `cargo +1.95.0` in run steps (lines 44, 169)
> - Change `rustup component add --toolchain stable` to `--toolchain 1.95.0` (lines 29, 142)
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> 🤖 Prompt for AI Agents
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> ```
> Verify each finding against the current code and only fix it if needed.
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> In @.github/workflows/test-cedarling.yml around lines 192 - 195, Update all
> Cedarling workflow steps that currently use the unpinned "stable" Rust toolchain
> to the pinned version "1.95.0": replace occurrences of toolchain: stable with
> toolchain: "1.95.0" in each job's with: block, change any run steps invoking
> cargo +stable to cargo +1.95.0, and change rustup component add --toolchain
> stable to --toolchain 1.95.0 so every job (including the python_tests and other
> Cedarling jobs) uses the exact same 1.95.0 toolchain.
> ```
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_Originally posted by @coderabbitai[bot] in [#13862](https://github.com/JanssenProject/jans/pull/13862/changes#r3109652210)_

Inspect .github/workflows/test-cedarling.yml and compare its Rust declarations with jans-cedarling/rust-toolchain.toml. Search the workflow for toolchain: stable, cargo +stable, and rustup component add --toolchain stable, then verify every Cedarling job uses 1.95.0 consistently.

Written by the indexing model from the issue text.