feat: support per-runner-flavor SQS batch size and window in multi_runner_config (#5108)

## Description

`lambda_event_source_mapping_batch_size` and
`lambda_event_source_mapping_maximum_batching_window_in_seconds` are
module-level variables applied identically to every runner flavor.
Deployments with mixed load profiles (e.g. 1000-runner high-volume vs
10-runner low-volume) need per-flavor tuning to avoid unnecessary
latency, wasted SSM throughput, and GitHub API rate limit pressure on
low-volume flavors.

Adds both as `optional(number, null)` fields in
`multi_runner_config.runner_config`, with `coalesce()` fallback to the
existing module-level variables. Follows the established pattern used by
`scale_up_reserved_concurrent_executions`.

**Changes:**
- `modules/multi-runner/variables.tf` — new optional fields in the
`runner_config` object type + descriptions; restored `iam_overrides`
fields accidentally dropped during merge
- `modules/multi-runner/runners.tf` — `coalesce(per_flavor,
module_level)` for both parameters; restored `iam_overrides` wiring
- `modules/multi-runner/README.md` — regenerated with terraform-docs to
reflect all changes; kept in sync with latest `main`

**Usage:**
```hcl
multi_runner_config = {
  runner-large = {
    runner_config = {
      lambda_event_source_mapping_batch_size                         = 50   # override
      lambda_event_source_mapping_maximum_batching_window_in_seconds = 10   # override
      # ...
    }
  }
  runner-metal = {
    runner_config = {
      # no override — uses module-level defaults
      # ...
    }
  }
}
```

Fully backwards compatible. Existing configs require no changes.

## Test Plan

- Verified with `terraform fmt` (clean)
- CodeQL security scan passed (no issues)
- Code review passed (no issues found)
- Confirmed `iam_overrides` is fully restored in `runner_config` object
type, heredoc description, standalone variable, and `runners.tf` wiring
- Merged latest `main` and regenerated README; no remaining conflicts

## Related Issues

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Signed-off-by: Brend Smits <brend.smits@philips.com>
Co-authored-by: Brend Smits <brend.smits@philips.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: 3 people