Reject non-numeric input in Length.TryParse

[jhaygood86]

Problem

Length.TryParse reports success for input that is not a length at all:

Length.TryParse("auto", out var r);   // true, r == 0px
Length.TryParse("red", out var r);    // true, r == 0px
Length.TryParse("", out var r);       // true, r == 0px
Length.TryParse("garbage", out var r) // true, r == 0px

The zero fallback exists for a good reason — CSS Values 3 §5.2 allows a zero length to omit its unit — but it keys only on the value:

var unitString = s.StylesheetUnit(out var value);
var unit = GetUnit(unitString);

if (unit != Unit.None) { result = new Length(value, unit); return true; }

if (value == 0f) { result = Zero; return true; }   // <-- also matches "auto", "red", ""

StringExtensions.StylesheetUnit sets result = default (0) both when it parses a unitless zero and when it can't find a number at all, so the two are indistinguishable by value alone.

Fix

They are distinguishable by the returned unit string — empty for a plain number, null when nothing could be tokenized — so check that too.

Angle, Frequency and Resolution have no equivalent fallback (they require a recognised unit), so this is specific to Length.

Tests

New LengthTests.cs, 19 theory cases:

• values with units, including 0px and negatives
• unitless zero in its accepted forms — 0, 0.0, -0
• non-length input that must now be rejected — "", auto, inherit, red, px, %, garbage
• a non-zero number without a unit (5, -2.5) and an unrecognised unit (10foo)

7 fail on master. The full suite (1263 existing tests) stays green, and all seven target frameworks build with no new warnings.