fix: std.format accepts boolean for numeric conversion codes

[He-Pin]

Summary

The official Jsonnet spec states:

The string formatting follows the same rules as Python.

In Python, bool is a subclass of int, so "%d" % True returns "1". However, jrsonnet currently rejects booleans for all numeric conversion codes with type error: expected number, got boolean.

This fix coerces Val::Bool(true) to Val::Num(1.0) and Val::Bool(false) to Val::Num(0.0) before numeric conversion in format_code(), matching Python's behavior.

The %s conversion is intentionally left unchanged, as Python's "%s" % True returns "True" (not "1").

Related upstream PRs: google/jsonnet#1328, google/go-jsonnet#884

Before / After

Expression	Python	jrsonnet (before)	jrsonnet (after)
"%d" % true	"1"	ERROR	"1"
"%f" % false	"0.000000"	ERROR	"0.000000"
"%x" % true	"1"	ERROR	"1"
"%o" % true	"1"	ERROR	"1"
"%e" % true	"1.000000e+00"	ERROR	"1.000000e+00"
"%g" % true	"1"	ERROR	"1"
"%c" % true	"\x01"	ERROR	"\x01"
"%s" % true	"True"	"true"	"true" (unchanged)

Test plan

• Existing format tests continue to pass (cargo test -p jrsonnet-evaluator -- 7/7 passed)
• Boolean coercion matches Python's bool -> int behavior for all numeric codes (%d, %o, %x, %e, %f, %g)
• Boolean coercion applied for %c (character code) matching Python
• %s conversion unaffected -- booleans still render as "true"/"false"