Fix GetString precision loss for large int64 values

[michaelglass]

Summary

• Fix for RouteCollectionReader.GetString converts large numeric strings to scientific notation #149: GetString now correctly returns the original string value for large integers instead of converting to scientific notation

Problem

GetString was converting large integer route values (like Int64.MaxValue) to scientific notation, causing precision loss:

ctx.Request.RouteValues.Add("id", "9223372036854775807")  // Int64.MaxValue
let route = Request.getRoute ctx
route.GetString "id"  // Was: "9.223372036854776E+18", Now: "9223372036854775807"

Root Cause

The IsFloat pattern was parsing all numeric-looking strings as float64 values. For integers with more than ~15 significant digits, this caused precision loss since float64 only has 15-16 digits of precision.

Solution

Modified the IsFloat pattern to not match integer strings with more than 15 significant digits. Such values now remain as RString, preserving the original value exactly.

This doesn't break GetInt64 because it already handles RString values by parsing them directly via StringParser.parseInt64.

Test plan

• Added unit tests for precision boundary (15 vs 16+ digit integers)
• Added regression test for route values with Int64.MaxValue
• All 71 existing tests pass

🤖 Generated with Claude Code.

I (the human, Michael) couldn't find any place in the contribution docs that was against this, I hope it's ok. (also: want to be transparent about our slow demise as working software devs)