chore: pin requirements with hashed lock file

[goharanwar]

Summary

• Split dependencies into requirements.in (loose constraints) and requirements.txt (pip-compile generated lock with hashes), resolved against Python 3.14.
• Update PR validation CI to Python 3.14 and add a check that fails if requirements.txt drifts from requirements.in.
• README: document how to regenerate the lock file.

Motivation

The platform repo's admin-ops service shells out to this test suite and currently does pip install -r requirements.txt at docker build time. Consumers that want hermetic, reproducible dependency resolution (e.g. Bazel pip.parse(requirements_lock = ...) which is how common/doc-conversion-py handles pymupdf) require a fully pinned lock file with hashes.

This change makes the suite compatible with that pattern while keeping pip install -r requirements.txt working for existing users — they just now get reproducible installs.

Test plan

• Lock file generated with pip-compile --generate-hashes --output-file=requirements.txt requirements.in inside a python:3.14-slim container
• pip install -r requirements.txt succeeds on Python 3.14 (verified via admin-ops Dockerfile build — 27 cp314 wheels installed cleanly)
• Sanity smoke profile (7 tests) passes end-to-end against api.vectara.dev
• CI "Verify requirements.txt is in sync with requirements.in" step passes on this PR (will show green on push)
• Existing pytest collection step still passes on Python 3.14

Follow-up

Once this merges, the platform repo admin-ops PR will be updated to bump the pinned commit SHA and (in a follow-up) wire up git_repository + pip.parse so the Dockerfile no longer clones the repo or runs pip install at build time.

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