Deduplicate shared preamble in expectation-value sweeps

[yurekami]

Fixes #4209.

What

DensityMatrixSimulator.simulate_expectation_values_sweep and
Simulator.simulate_expectation_values_sweep_iter began with an identical preamble
(the one flagged by the # TODO(#4209) comment): reject terminal measurements unless
permitted, resolve the qubit order, build the qubit→index map, and wrap the observables
as cirq.PauliSums.

This factors that preamble into a protected helper _qubit_map_and_pauli_sums on
SimulatesExpectationValues (the base both simulators already inherit) and removes the
# TODO(#4209) marker.

Design context

Only the preamble is shared; the two methods' bodies are intentionally left as they are.
The base class documents "Prefer overriding simulate_expectation_values_sweep_iter", and
the sparse Simulator deliberately stays a lazy generator that streams over
simulate_sweep_iter and uses expectation_from_state_vector, while
DensityMatrixSimulator is eager and uses expectation_from_density_matrix. Collapsing the
two into one method would regress the sparse path's streaming behavior, so the helper covers
only the common setup. The helper returns the already-resolved QubitOrder so the qubit map
and the downstream simulate(...) call use the same ordering — no behavior change.

No public API change (the helper is protected); no serialization changes.

Scope / non-goals

• Does not change the eager-vs-lazy structure of either method.
• Does not touch the other Simulates* interfaces or the Clifford/MPS simulators (they
  don't override these methods).
• No new public symbols.

Test

No new behavior is introduced, so the existing expectation-value tests act as the regression
net — density_matrix_simulator_test.py, sparse_simulator_test.py, and simulator_test.py
all pass (473 tests), and they already cover the numeric expectation values, the
terminal-measurement ValueError, and the permit_terminal_measurements flag, which
exercise every line of the new helper.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.60%. Comparing base (b29b6d4) to head (a234874).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8123      +/-   ##
==========================================
- Coverage   99.60%   99.60%   -0.01%     
==========================================
  Files        1118     1118              
  Lines      100957   100954       -3     
==========================================
- Hits       100557   100553       -4     
- Misses        400      401       +1     

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