Add diagnosis APIs to MediaWiki benchmark

[marziehlenjaniMeta]

Summary:
This diff adds reusable pre-flight diagnostic checks and centralizes diagnosis
merging at the framework level, so all benchmarks benefit automatically.

1. Shared check functions (packages/common/diagnosis_utils.py)
Added two reusable check functions to the shared diagnosis API:

• check_file_descriptor_limit(): verifies ulimit is sufficient, with optional
  auto-fix that records the change under notable_auto_fixes
• check_selinux(): detects SELinux Enforcing mode (causes HHVM segfaults)

These join existing check_ipv6_hostname() and check_port_available().

2. Generic pre-flight CLI (packages/common/preflight_checks.py)
A reusable script callable from any benchmark's bash runner:

• Accepts --benchmark <name> and --min-fds <N> for per-benchmark config
• Runs SELinux, IPv6 hostname, and file descriptor checks
• Fixes inverted check_ipv6_hostname usage (returns True when IPv6 is
  detected, not necessarily broken — should not gate all_ok)

3. Framework-level diagnosis merge (benchpress/cli/commands/run.py)
After job.run(), run.py now automatically merges DiagnosisRecorder records
(failures + auto-fixes) into metrics via DIAGNOSIS_FILE_PATH. Every benchmark
gets diagnosis merging for free — no per-benchmark merge code needed. Removed
the now-redundant merge_failure_to_results() call from tao_bench's
run_autoscale.py.

4. MediaWiki integration

• run.sh: Calls common preflight_checks.py before benchmark; raises
  ulimit -n in bash so child processes (HHVM, nginx, wrk) inherit the limit
• jobs.yml: Added -U {auto_fix_ulimit} arg with default auto_fix_ulimit=1
  to all 5 mediawiki job entries

Differential Revision: D98805269

[meta-codesync]

@marziehlenjaniMeta has exported this pull request. If you are a Meta employee, you can view the originating Diff in D98805269.