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#### What I did
Aligned the ERSPAN mirror session `sample_rate` validation range with the YANG model ([sonic-buildimage #26756](sonic-net/sonic-buildimage#26756)), which defines it as uint32 `2..max`.

#### How I did it
 - `config/main.py`: changed from `SAMPLE_RATE_MIN` 256 to 2, changed `SAMPLE_RATE_MAX` 8388608 to `0xFFFFFFFF` (uint32 max).
 - `tests/config_mirror_session_test.py`: updated assertions and reworked the boundary test.
 - `doc/Command-Reference.md`: updated the valid range.

#### How to verify it
 pytest tests/config_mirror_session_test.py

Signed-off-by: Sonic Build Admin <sonicbld@microsoft.com>

#### Previous command output (if the output of a command-line utility has changed)

#### New command output (if the output of a command-line utility has changed)