Add is_backtracking and is_at_limit outputs to singleaxis

[r0hansaxena]

• Closes Add “is it true tracking” Boolean output to singleaxis #2672
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Add two boolean output columns to pvlib.tracking.singleaxis:

• is_backtracking: True when tracker is in backtracking mode
• is_at_limit: True when rotation angle is clipped to max/min

As discussed in #2672, this implements the simpler suggestion of adding two boolean flags (is_backtracking and is_at_limit) indicating the physical state of the tracker, rather than a full enumerated state dictionary.

• is_backtracking is determined by whether the tracker is actively avoiding row-to-row shade (temp < 1 when backtracking is enabled).
• is_at_limit is determined by comparing the clipped tracker_theta to its unclipped ideal angle.
  Existing tests were updated to gracefully handle the extra columns without breaking, and two new tests were added specifically to cover the new boolean flags.