Document inline dependency controls

src/content/docs/guides/testing/create-fixtures.mdx

@@ -170,13 +170,18 @@ def localstack():
     ...
 ```
 
-When a fixture file declares dependencies, `tenzir-test` installs them with
-`uv` before importing project fixtures:
+When a fixture file declares dependencies, `tenzir-test` installs missing
+packages into the active Python environment with `uv` before importing project
+fixtures:
 
 ```sh
 uv pip install --python <current-python> boto3
 ```
 
+If a bare dependency name, such as `boto3`, is already installed, the harness
+reuses it instead of running `uv pip install`. Versioned requirements, such as
+`boto3>=1.34`, still use `uv` so the requested version is enforced.
+
 This works for `fixtures.py` and for any Python module under `fixtures/`,
 including nested modules imported by `fixtures/__init__.py`. The feature also
 applies in standalone fixture mode, such as `uvx tenzir-test --fixture
@@ -187,6 +192,12 @@ fixture imports an undeclared package, `tenzir-test` still reports the missing
 dependency and suggests running the harness from a project environment or with
 `uvx --with`.
 
+Pass `--disable-inline-dependency-install`, or set
+`TENZIR_TEST_DISABLE_INLINE_DEPENDENCY_INSTALL=1`, when another tool, such as
+Nix, provisions all fixture dependencies and `tenzir-test` should not install
+packages at runtime. The harness still reads dependency metadata, but fixture
+imports can fail if the dependency is not actually available.
+
 ## Use container runtime helpers
 
 When a fixture manages a single container directly rather than orchestrating

src/content/docs/reference/test-framework/index.mdx

@@ -204,6 +204,9 @@ Useful options:
 - `--no-hooks`: Disable project hook loading and invocation. Use this when you
   need to recover from a broken hook or compare behavior without hook side
   effects.
+- `--disable-inline-dependency-install`: Disable runtime installation for
+  inline Python dependencies declared by tests or fixtures. Use this when
+  another tool provisions the Python environment.
 
 Set `TENZIR_TEST_DEBUG=1` in CI when you want the same diagnostics without
 passing `--debug` on the command line.
@@ -1229,6 +1232,9 @@ walks through the full implementation.
   `--debug`).
 - `TENZIR_TEST_DISABLE_HOOKS` – Disable project hook loading and invocation
   (equivalent to `--no-hooks`).
+- `TENZIR_TEST_DISABLE_INLINE_DEPENDENCY_INSTALL` – Disable runtime
+  installation for inline Python dependencies declared by tests or fixtures.
+  Equivalent to `--disable-inline-dependency-install`.
 
 ### Binary resolution