faster-redis: reach pipeline and transaction parity with redis-py

[justrach]

Bring Pipeline behavior closer to redis-py for command buffering, transactions, and result handling.

Scope:

• Ensure fallback commands and explicit methods behave consistently inside pipelines.
• Match transaction wrapping and result semantics for MULTI/EXEC flows.
• Audit error propagation and mixed-result handling.
• Evaluate gaps for WATCH, optimistic locking, and pipeline state-reset behavior.
• Add parity tests for both transactional and non-transactional pipelines.

Why:
Pipeline semantics are one of the easiest areas to misrepresent with simple smoke tests and one of the highest-value compatibility surfaces.

[justrach]

Blocked by: #69.

[justrach]

Blocked by: #70.