Add infer option to changelog bundle promotion

[lcawl]

Relates to elastic/docs-builder#3906

Summary

Adds the --infer functionality to the appropriate GitHub Actions. Here's what was done:

1. changelog/bundle-create/action.yml - Added infer input

• Added new input infer (default: 'false') with description explaining it enables PR metadata synthesis when no pool file matches
• Added INFER environment variable to the validation step
• Added validation to ensure infer is only used with start-git-ref/end-git-ref
• Added INFER environment variable to both the "Resolve bundle plan" and "Generate changelog bundle" steps
• Appended [ "$INFER" = "true" ] && ARGS+=(--infer) to both the plan step and the generate step to pass the flag to docs-builder

2. changelog-promotion-bundle.yml - Added infer input and forwarding

• Added new workflow input infer (type: boolean, default: false) with documentation
• Updated the comment example to show how callers can pass the optional infer parameter
• Updated the bundle-create action call to forward infer: ${{ inputs.infer }}

The implementation follows the overall plan:

• CDN-only source is now the default (unmatched PRs warned/omitted like --prs)
• Users opt-in via infer: true in the action input or bundle.profiles.<name>.infer_missing_changelogs: true in YAML
• The flag is only valid with start-git-ref/end-git-ref (commit-range bundling)
• Both the plan and generate steps properly pass --infer to docs-builder when enabled

NOTE: Though no one is using these git-ref-based actions yet, they would nonetheless NOT need to make changes unless they want to opt into the new synthesis functionality.