[Router] Add prefix_min_match_length threshold to PrefixAwareRouter (#959)

Add a configurable --prefix-min-match-length option for the
prefixaware routing logic. When the longest prefix match is shorter
than this threshold, the request falls back to QPS-based routing
instead of using the matched endpoint, mitigating load hotspots
caused by long shared prefixes (e.g. common system prompts).

Defaults to 0, which preserves the original behavior.

Partially addresses #957.

Signed-off-by: Max Li <hitliqiwei@gmail.com>
Co-authored-by: Rui Zhang <51696593+ruizhang0101@users.noreply.github.com>