chore(deps): generate Gemfile.lock files and allow in .gitignore for Phase 2 sample

[torreypayne]

Overview

This pull request introduces `Gemfile.lock` tracking for a representative pilot sample of Phase 2 (`Batch 2: Generated Gems A - D`) client libraries, expanding upon the Phase 1 rollout (PR #34666) and fulfilling the P1 dependency security mandate (b/509981628).
By checking in these lockfiles, we eliminate non-deterministic dependency resolutions during CI presubmit/postsubmit runs and enroll these libraries into weekly automated lockfile maintenance (`bundle lock --update`) managed by Renovate.

Enrolled Scope (5 Sample Libraries)

To safely validate `.gitignore` negation and lockfile generation behavior across the major Generated Gem prefix families before scaling across all ~198 Phase 2 gems, this PR enrolls the following representative sample:

• `google-ads-ad_manager-v1`
• `google-analytics-data-v1beta`
• `google-apps-chat-v1`
• `google-area120-tables-v1alpha1`
• `google-cloud-bigquery-data_exchange-v1beta1`

Changes Included

1. `.gitignore` Negation Rules: Appended `# Allow tracking Gemfile.lock for Phase 2 rollout\n!Gemfile.lock` to each gem's `.gitignore` file so that `Gemfile.lock` is tracked by Git while preserving general build/doc ignore patterns.
2. Initial Lockfile Generation: Executed `bundle lock` inside each of the 5 gem directories using Ruby 3.3 and Bundler to generate deterministic, production-ready `Gemfile.lock` files.

Verification

• Checked `git status` to verify that `Gemfile.lock` files changed from ignored (`!!`) to tracked (`??` / staged `A`).
• Confirmed dependencies resolve cleanly against local monorepo sibling path specifications (e.g., `gapic-common`, `google-cloud-errors`) and `https://rubygems.org/\` for third-party gems.

Related Issues

• Resolves b/509981628 (Phase 2 Sample Pilot)