Remove em-dashes from served discovery copy (2.7.2)

[vvillait88]

Summary

Mirror of the Node SDK change, string for string, so both SDKs keep rendering the same shared identity and payment blocks. Every string these builders emit onto a merchant's llms.txt, skill.md, OpenAPI document and 402 body is now free of em-dashes, which the house style bars on external surfaces.

Two worth calling out:

• The empty compatible-clients table cell rendered a bare em-dash as its placeholder. It now reads none, matching the Node SDK that renders the same table.
• Five docstrings said "agent-commerce". The house category term is "agentic commerce".

Docstrings and comments are otherwise untouched: internal prose, never served, and they carry the rationale. Note that Python docstrings are not automatically published here the way a FastAPI route docstring is, so the served set is just the string literals in the builders.

Type of change

• Bug fix (no breaking change)
• New feature (no breaking change)
• Breaking change (existing callers must update)
• Docs, tests, or internal maintenance only

Public API

No signature, shape, or export changes. The rendered TEXT of discovery surfaces changes, which is the point. A consumer asserting on the exact old wording of a shared identity or payment line would need to update; nothing else is affected. Version bumped 2.7.1 to 2.7.2, patch, since client impact is text only.

Test plan

uv run ruff check ., uv run ruff format --check ., uv run pytest: 1863 passing, 4 skipped, coverage 95.37% against the 95% floor.

Cross-checked against the Node SDK line by line so the two do not drift: the same twenty-one strings, the same replacements, the same none placeholder. One string in this copy was initially missed that the Node copy had fixed (the SPT rail identity-gating line); caught by diffing the two builders against each other rather than by re-reading either one alone.

Checklist

• Tests cover the new behavior, and the suite passes locally
• Lint, format, and type checks pass
• Docs and README examples updated if the public surface changed — not applicable, the public surface is unchanged
• No secrets, credentials, or personal data in the diff or the tests