Parser duplicates the prefix-modifier range rule instead of using UriTemplateVarSpec constants

[brenpike]

Spun out of #30 so that tracking issue can be closed. #30's own acceptance criteria are met; this was listed there as deferred follow-up work — and the duplication it anticipated did in fact materialize.

Problem

The prefix-modifier range rule is expressed twice.

UriTemplateVarSpec defines the bounds as named constants and validates against them:

• src/Chatter.Rest.UriTemplates/UriTemplateVarSpec.cs:39 — internal const int MaxPrefixLength = 9999;
• :99 — if (length < MinPrefixLength || length > MaxPrefixLength)

UriTemplateParser re-implements the same rule with hardcoded literals:

• src/Chatter.Rest.UriTemplates/UriTemplateParser.cs:134 — if (!int.TryParse(suffix, out var prefixLen) || prefixLen < 1 || prefixLen > 9999)
• :112 and :137 — the bounds are also spelled out in the message text

The two agree today. Nothing enforces that they keep agreeing.

Requested change

Have UriTemplateParser use UriTemplateVarSpec.MinPrefixLength / MaxPrefixLength rather than literals, and interpolate them into the exception messages so the text cannot drift from the check either.

Behavior-preserving refactor; no observable change, no version bump beyond whatever the touched src/ path already requires per CLAUDE.md.