docs(a2a): drop drifting line numbers from store method docs

[FanouZeng-TT]

Description

Three A2A docs pin store.py methods to fixed source line numbers that have
drifted as a2a/business_agent/src/business_agent/store.py evolved:

Doc	Reference	Documented	Actual line
01-architecture.md	start_payment()	463	452
01-architecture.md	place_order()	498	484
04-commerce-flows.md	_get_fulfillment_options()	525	511
04-commerce-flows.md	place_order()	498	484
06-extending.md	start_payment()	463	452
06-extending.md	_get_fulfillment_options()	525	511

The 01-architecture.md "Key Methods" table also lists a Line column whose
add_to_checkout (186 vs 183) and get_checkout (244 vs 241) values are
already wrong and will keep advancing. Line numbers are volatile navigation
state; the method names and file path already locate the implementation.

Fix: remove the drifting line markers while keeping the method names and
store.py file reference, so the docs stop navigating readers to the wrong
source lines.

Category (Required)

• Core Protocol: Protocol specification changes. (Requires Technical Council approval)
• Governance/Contributing: Governance or contribution process changes. (Requires Governance Council approval)
• Capability: Capability specification changes. (Requires Maintainer approval)
• Documentation: Documentation updates. (Requires Maintainer approval)
• Infrastructure: Build, CI, tooling, or release changes. (Requires DevOps Maintainer approval)
• Maintenance: Dependency, compatibility, or routine maintenance changes. (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. (Requires Maintainer approval)
• Community Health (.github): Repository-wide health files and configuration. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide.
• 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)

rg -n '^ def ' store.py confirms current definitions (452/484/511); no
store.py:<line> or (line N) references remain for these methods.
uvx pre-commit run --all-files: passed
git diff --check: passed