Fix: propagate MutuallyExclusiveFlags persistent flags to subcommands

[siutsin]

What type of PR is this?

• bug

What this PR does / why we need it:

• Fixes persistent flag propagation for flags defined in MutuallyExclusiveFlags groups
• Changed pCmd.Flags to pCmd.allFlags() in the persistent flag inheritance loop in command_parse.go:39, so that flags from MutuallyExclusiveFlags are included when walking the command lineage
• Added test TestCommand_ExclusiveFlagsPersistentPropagation to verify the fix

Which issue(s) this PR fixes:

Fixes #2265

Special notes for your reviewer:

allFlags() is already used on line 29 of the same function to populate cmd.appliedFlags for the current command. The ancestor loop on line 39 was the only place still using pCmd.Flags directly, which excluded MutuallyExclusiveFlags groups from propagation.

Testing

• Added TestCommand_ExclusiveFlagsPersistentPropagation which creates a root command with a MutuallyExclusiveFlags group containing persistent string flags, then verifies a subcommand can receive the flag value
• All existing tests continue to pass

Release Notes

Fixed persistent flags defined in `MutuallyExclusiveFlags` not being propagated to subcommands