chore(bigtable): correct sparse-bootstrap dep discovery and install ordering

[igorbernstein2]

The previous script missed cross-project SNAPSHOT deps and could fail on clean-m2 runs. Fixes:

• Walk regular edges (not just parent + BOM imports) so monorepo SNAPSHOTs like api-common-java are discovered.
• Version-aware edge following: skip edges whose declared version doesn't match the local pom, so pinned released BOM imports (e.g. google-cloud-pubsub-bom:1.140.0 from google-cloud-jar-parent) aren't mistakenly followed to the local SNAPSHOT, which created cycles and phantom projects in the install list.
• Verify the local pom's coord+version matches before accepting the shortcut; the Maven default '../pom.xml' otherwise resolves to the unrelated repo-root pom.
• Enqueue each project's aggregator /pom.xml when any pom under it is visited, since 'cd && mvn' forces Maven to load the aggregator regardless of -pl.
• Flush the install batch on every top-level project change so interleaved topo orders (A1, A2, B1, A3 when A3 depends on B1) install in dep order instead of being collapsed per project.
• Match pom filenames by exact basename so test resources like src/test/resources/gax-example-pom.xml aren't picked up as modules.

[gemini-code-assist]

Code Review

This pull request enhances the sparse-bootstrap.py script to resolve and traverse regular <dependency> edges within the monorepo, and updates the install command generation to preserve topological order by grouping consecutive POMs of the same project. The review feedback highlights three key improvements: resolving a namespace issue in get_version that prevents inheriting versions from parent POMs, adding the -N flag to mvn install when no sub-modules are present to prevent recursive builds in sparse checkouts, and caching parsed XML roots to avoid redundant parsing during dependency resolution.