Add voltage angles to observation vector

[Gaurav890]

Summary

• add theta_or, theta_ex, load_theta, gen_theta, and storage_theta to the complete observation vector
• append the angle fields so existing observation-vector offsets remain unchanged
• preserve the pre-1.12.6 vector layout when loading older serialized observations
• document the unsupported-backend zero convention and update regression expectations

Why

Voltage-angle attributes were only present in attr_list_json, so to_vect() omitted them and from_vect() could not reconstruct them. Backends that do not expose voltage angles remain supported: Grid2Op already fills these arrays with zeros and reports their availability through support_theta.

Fixes #711.

Validation

• python -m unittest grid2op.tests.test_Observation — 83 passed, 1 skipped
• python -m unittest grid2op.tests.test_attached_envs_compat — 26 passed
• python -m unittest grid2op.tests.test_Runner — 19 passed
• python -m unittest grid2op.tests.test_GymConverter — 20 passed
• focused attached-environment observation-size tests — 4 passed
• pre-commit secret scan and mailmap check — passed

[codacy-production]

Not up to standards ⛔

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