Python: enforce dependency-bounds validator in CI

[vaibhav-patel]

Fixes #6582.

Motivation & Context

The dependency-bounds validator (uv run poe validate-dependency-bounds-test) is
only run by the weekly python-dependency-maintenance.yml workflow, where it is
continue-on-error: true and merely opens an issue on failure. It is not a required
PR check, so dependency-floor, missing-optional-dependency, and isolated-env typing
regressions can land on main and ship in a release before anyone notices — as
happened at the python-1.8.1 tag, which was released while the validator was red
for packages/core at lowest-direct resolution.

Description & Review Guide

Adds a blocking Dependency Bounds Validation job to python-code-quality.yml,
which already runs on pull_request → main, merge_group, and workflow_dispatch.
The job runs validate-dependency-bounds-test --package "*", which smoke-tests both
ends of every package's allowed dependency range (lowest-direct and highest) and
runs each package's pyright pass in an isolated environment.

• No continue-on-error, so it can gate PRs and the merge queue — the gap this
  issue is about.
• Mirrors the dependency maintenance workflow's environment (Python 3.13, full git
  history for the internal workspace graph) so PR results line up with the nightly sweep.
• Uploads the JSON report as an artifact for triage on failure.
• timeout-minutes: 60 guards against a hung resolver.

The underlying floor/packaging issues this validator caught (the telemetry
find_spec except clause, the agent-framework-tools dev dependency on core, and
the shell-tool test skipif guards) were already fixed in the 1.9.0 version bump, so
the validator is green on main and this job can be required immediately.

Cost / scoping note

A full lower+upper sweep across all packages is somewhat slow. If per-PR cost becomes
a concern, follow-up options are to scope the run to changed packages on standard PRs,
or to run the full sweep nightly plus required-on-merge-queue. This PR wires up the
full required check first as the minimal correct fix; scoping can be layered on later.

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[Copilot]

Pull request overview

This PR adds a new required CI job to run the Python dependency-bounds validator on pull requests / merge queue events, so dependency floor/ceiling and isolated-environment typing regressions are caught before merging to main.

Changes:

• Add a blocking Dependency Bounds Validation job to the existing Python code-quality workflow.
• Run uv run poe validate-dependency-bounds-test --package "*" with a 60-minute timeout and upload the JSON results artifact for triage.

[vaibhav-patel]

Good catch — set UV_EXCLUDE_NEWER in the dependency-bounds job to match the maintenance workflow's 7-day cutoff in 6c45fdc.