feat: Introduce Location Search + Lookup capabilities

[jingyli]

Description

Defines standard interfaces for discovering, searching, and retrieving physical locations (e.g., retail stores, restaurants, warehouses, lodging properties).

It introduces two new capabilities under the dev.ucp.common namespace (consistent with Shopping's Catalog capability design):

1. Location Search (dev.ucp.common.location.search): Discovery-focused endpoint for natural language query, geographic, and offerings-based filters.
2. Location Lookup (dev.ucp.common.location.lookup): Resolution-focused endpoints supporting single & batch lookups.

Some key commerce flows it will be able to unlock:

• Local Pickup Discovery: Finding locations like retail stores or restaurant branches
  nearby that support customer pickup and checking their operating hours & inventory availability
  before selection.
• Fulfillment Area Verification: Checking if a specific location (e.g., utility depot, restaurant,
  or local service provider) has delivery coverage for a buyer's address.

Category (Required)

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

This is related to RFC #375's section 10.

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)

TBD