fix(yaml): quote leading-zero integer-shaped strings in stringify

[LeSingh1]

Fixes #7040.

resolveYamlInteger accepts "07" as octal so stringify already quoted it, but "08" and "09" fell through because they aren't valid octal:

import { stringify } from "jsr:@std/yaml";
stringify("07"); // "'07'\n"  ok
stringify("08"); // "08\n"    looks like a number
stringify("09"); // "09\n"    looks like a number

These still read as numbers to humans and to other YAML parsers, so they should be quoted too. The fix adds a /^[-+]?0[0-9]+$/ guard in chooseScalarStyle so any integer-shaped string with a leading zero forces single-quoted style. After the patch:

"08"   -> "'08'\n"
"09"   -> "'09'\n"
"089"  -> "'089'\n"
"-08"  -> "'-08'\n"
"0a8"  -> "0a8\n"   (unchanged, not integer-shaped)

Tests added in yaml/stringify_test.ts cover the issue cases plus edge cases (signed, mixed digits, non-integer-shaped strings).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (cdf74a8) to head (06cc8f2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7164      +/-   ##
==========================================
- Coverage   94.57%   94.57%   -0.01%     
==========================================
  Files         636      636              
  Lines       52142    52144       +2     
  Branches     9401     9402       +1     
==========================================
+ Hits        49315    49316       +1     
  Misses       2249     2249              
- Partials      578      579       +1     

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