fix(cli): keep commas in --attribute values

[RaphaelFakhri]

Fixes #1003

Summary

--attribute KEY=VALUE is a string slice flag. urfave/cli splits slice flag values on commas unless the command sets DisableSliceFlagSeparator. The commands below did not set it, so a value that contains a comma is split into separate pairs:

lk token create --attribute tags=a,b

parseKeyValuePairs then receives tags=a and b, and the result is {"tags": "a", "": "b"}. The value is truncated and a bogus attribute with an empty key is added.

This change sets DisableSliceFlagSeparator on the commands that read --attribute, following the existing handling of secrets and agent attributes:

• lk token create and the deprecated lk create-token
• lk room join
• lk perf agent-load-test

Repeat the flag to pass more than one attribute.

These commands also accept comma-separated values for --allow-source (for example camera,microphone). Because the separator is now disabled, lk token create splits those values itself in parseAllowSources.

Testing

• Adds TestAttributeFlagKeepsCommasInValues, which finds every command in the token, room, and perf trees that defines --attribute and parses --attribute tags=a,b --attribute mode=x with that command's flags and separator setting.
• The test fails for all four commands on main and passes with this change.
• Adds TestParseAllowSources for the comma-separated --allow-source values.
• go test ./cmd/lk passes.

[CLAassistant]

All committers have signed the CLA.