Add VALID_MODES constant and validate tracker mode at initialization

[JuanVqz]

Description

Introduces DeprecationTracker::VALID_MODES as a single source of truth for accepted tracker modes (save, compare). Constants live in lib/deprecation_tracker/valid_modes.rb so the exe/deprecations CLI can load them without pulling in the full library.

Motivation and Context

Valid mode values were duplicated as inline literals across lib/deprecation_tracker.rb and exe/deprecations. Adding or removing a mode required updating every literal. With VALID_MODES, one change propagates to all validation, error messages, and CLI help text automatically.

Additionally, passing an invalid mode previously silently no-oped (neither save nor compare ran after the test suite). It now raises ArgumentError early, covering both the library and CLI entry points.

How Has This Been Tested?

• Updated existing test that expected silent no-op on invalid mode to now expect ArgumentError
• Added test for invalid ENV['DEPRECATION_TRACKER'] value via init_tracker
• Added VALID_MODES describe block: constant membership and all valid modes construct without error
• Full spec suite passes (bundle exec rspec spec/deprecation_tracker_spec.rb)

Screenshots:

N/A

I will abide by the code of conduct

[etagwerker]

@JuanVqz I think this might introduced an unwanted side effect (?)

bin/deprecations run without --tracker-mode is probably broken. The CLI builds:

command = "DEPRECATION_TRACKER=#{tracker_mode} #{rspec_command} ..."

When tracker_mode is nil, the spawned child sees ENV["DEPRECATION_TRACKER"] = ""

Empty string is truthy in Ruby, so:

mode = opts[:mode] || ENV["DEPRECATION_TRACKER"] || :save  # => ""
@mode = mode ? mode.to_sym : :save                          # => :""
# new check raises ArgumentError

Before this PR that silently no-op'd in after_run. After this PR it crashes at construction.

[JuanVqz]

@arielj addressed latest code changes 👍

[etagwerker]

Should this guard skip the case where no --tracker-mode is passed? --tracker-mode is optional, so on the documented default deprecations run (README + the help examples) opts[:tracker_mode] is nil, and valid_mode?(nil) is falsey, so this aborts with Invalid --tracker-mode nil. before reaching rspec.

I cloned the branch and confirmed it: deprecations run (no --tracker-mode) exits 1 on this branch, whereas on main it falls through to DEPRECATION_TRACKER= bundle exec rspec ... and the library treats the blank value as the default save. That blank-as-default path is exactly what sanitize_mode adds for the library, but the CLI guard rejects nil/blank before that logic runs.

What about only validating when a mode was actually supplied, so a genuine typo (--tracker-mode bogus) still fails loudly but the default keeps working?

Suggested change

	tracker_mode = opts[:tracker_mode]
	unless DeprecationTracker.valid_mode?(tracker_mode)
	abort "Invalid --tracker-mode #{tracker_mode.inspect}. Must be one of: #{DeprecationTracker.valid_modes_display}."
	end
	tracker_mode = DeprecationTracker.sanitize_mode(opts[:tracker_mode])
	if tracker_mode && !DeprecationTracker.valid_mode?(tracker_mode)
	abort "Invalid --tracker-mode #{tracker_mode.inspect}. Must be one of: #{DeprecationTracker.valid_modes_display}."
	end

[etagwerker]

Just one last comment