fix(mlflow): mint each run in its own workflow's experiment

[HackedRico]

Every execution after the first failed with:

Cannot start run with ID <id> because active experiment ID does not match
environment run ID.

Cause

mcp_svc minted every run into caldera-mcp-client-1 regardless of which
workflow was executing:

_exp = mlflow.get_experiment_by_name("caldera-mcp-client-1")

But author sets the process-wide active experiment to a different name,
caldera-mcp-FACTORY-client-1. MLflow refuses to resume a run whose experiment
differs from the active one (mlflow/tracking/fluent.py):

if (_active_experiment_id is not None
        and _active_experiment_id != active_run_obj.info.experiment_id):
    raise MlflowException(f"Cannot start run with ID {existing_run_id} ...")

So once an author run had switched the active experiment, every later
mlflow.start_run(run_id=...) raised.

Why it appeared to work once

Deferring set_experiment out of import time exposed it rather than causing it.
The first request runs before the switch and matches, so the failure starts on
the second request. Before the deferral both workflow modules called
set_experiment at import and whichever imported last won, which happened to
match the hardcoded name.

Fix

Workflow declares its experiment and mcp_svc mints there, so there is one
source of truth per workflow. The two experiments stay deliberately separate.

mlflow_experiment: str = "caldera-mcp-client-1"

Tests

tests/test_mlflow_experiment_binding.py covers the binding, including a test
that pins the MLflow mechanism itself so a future refactor reintroducing a
hardcoded name fails here rather than in production.

Verified the tests catch the regression: reverting the fix gives 1 failed, 4 passed, restoring it gives 5 passed.

Baseline on main is 51 passed across test_author_guards, test_config and
test_set_config_secrets; this branch is 56 with the 5 added. No regressions.

Tests need CALDERA core importable:

PYTHONPATH=<caldera-root> python -m pytest plugins/mcp/tests/test_mlflow_experiment_binding.py -v