validator: Deprecate --dev-halt-at-slot

[steviez]

Problem

This was a debug flag to stop the validator midway through startup. The argument no longer has value so start the removal process by marking it deprecated. The flag was first added prior to 8b2327e (2019); I didn't care to go digging further but its time has come

Summary of Changes

Move the argument to deprecated list + add a CHANGELOG entry

Verified the flag can still be set:

$ cargo run -- --dev-halt-at-slot 20 --identity test-ledger/validator-keypair.json --log - 
...
--dev-halt-at-slot is deprecated.  --dev-halt-at-slot will be removed in the future.
[2025-11-22T21:10:12.559089000Z INFO  agave_validator::commands::run::execute] agave-validator 4.0.0 (src:144249c9; feat:3706929056, client:Agave)
[2025-11-22T21:10:12.559229000Z INFO  agave_validator::commands::run::execute] Starting validator with: ArgsOs {
        inner: [
            "/.../src/solana/target/debug/agave-validator",
            "--dev-halt-at-slot",
            "20",
            "--identity",
            "test-ledger/validator-keypair.json",
            "--log",
            "-",
        ],
    }

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.6%. Comparing base (8e68644) to head (0fafa4d).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9232   +/-   ##
=======================================
  Coverage    82.6%    82.6%           
=======================================
  Files         890      890           
  Lines      320854   320854           
=======================================
+ Hits       265226   265262   +36     
+ Misses      55628    55592   -36     

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[t-nelson]

thanks for splitting!

[mergify]

Backports to the beta branch are to be avoided unless absolutely necessary for fixing bugs, security issues, and perf regressions. Changes intended for backport should be structured such that a minimum effective diff can be committed separately from any refactoring, plumbing, cleanup, etc that are not strictly necessary to achieve the goal. Any of the latter should go only into master and ride the normal stabilization schedule. Exceptions include CI/metrics changes, CLI improvements and documentation updates on a case by case basis.