fix(convert): zero-pad each channel in RgbToHex

[dualfroz]

Problem

RgbToHex (and its alias Rgb2hex) does not zero-pad channel values below
0x10, so a single hex digit is emitted for those channels:

color.RgbToHex([]int{1, 2, 3})   // "123"    , want "010203"
color.RgbToHex([]int{0, 0, 0})   // "000"    , want "000000"
color.RgbToHex([]int{10, 11, 12}) // "0a0b0c" -> actually "abc"

This breaks two contracts:

1. The round-trip with HexToRgb is no longer an identity for colors with any
   channel below 16:

   color.RgbToHex(color.HexToRgb("010203")) // "123", not "010203"

2. Rgb2basic looks a color up in hex2basicMap, whose keys are all
   fixed-width 6-digit hex strings (e.g. "000000"). Because RgbToHex(0,0,0)
   returns "000", pure black never matches the fast-path map and silently falls
   through to the approximate RgbToAnsi.

The sibling converter RgbTo256 already builds its hex key with
fmt.Sprintf("%02x%02x%02x", ...), so this is an inconsistency within the same
file.

Root cause

convert.go, RgbToHex:

for _, v := range rgb {
	hexNodes = append(hexNodes, strconv.FormatInt(int64(v), 16))
}

strconv.FormatInt(_, 16) produces the minimal representation, i.e. one digit
for values 0-15.

Fix

Format each channel as a fixed two-digit hex value:

hexNodes = append(hexNodes, fmt.Sprintf("%02x", v))

fmt is already imported in the file. Existing values >= 16 are unchanged.

Test

Extended TestRgbToHex in utils_test.go with small-channel cases
({0,0,0}, {1,2,3}, {10,11,12}) and an explicit
RgbToHex(HexToRgb(hex)) == hex round-trip check.

• Gate: go test ./ -> ok (PASS)
• go vet ./... -> exit 0
• go build ./... -> exit 0

Counterfactual: reverting RgbToHex to strconv.FormatInt(int64(v), 16) makes
the new test fail with expect: "abc" vs actual: "0a0b0c".

Note: gofmt -l convert.go reports the file, but this is pre-existing (a doc
comment near line 866, unrelated to this change and present on the base commit).
The changed region is gofmt-clean, so unrelated lines were intentionally left
untouched.