[stage2] fix: serialize multiline descriptions in legacy TOML renderer

[RbBtSn0w]

Summary

Fix the legacy TOML rendering path still used by preset and extension command registration.

This PR corrects the CommandRegistrar.render_toml_command() behavior for multiline description values so the generated TOML remains parseable and preserves the description text when loaded back through tomllib.

What changed

• render TOML description values through safe basic-string escaping instead of embedding raw newlines in a one-line TOML string
• add regression coverage for multiline description round-tripping in tests/test_extensions.py
• leave prompt fallback behavior unchanged

Validation

• uv run --with pytest python -m pytest tests/test_extensions.py -q -k "multiline_description or embedded_triple_double_quotes or escapes_when_both_triple_quote_styles_exist"
• uv run --with pytest python -m pytest tests/test_extensions.py -q

Issue

Part of #2095.

[Copilot]

Pull request overview

Fixes the legacy TOML rendering used during preset/extension command registration so multiline description values produce valid TOML and round-trip correctly through tomllib.

Changes:

• Escape TOML description values using a basic-string renderer (no raw newlines in one-line TOML strings).
• Add a regression test ensuring multiline descriptions parse and preserve semantics when loaded.
• Introduce a small helper for rendering TOML basic strings.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/specify_cli/agents.py	Updates legacy TOML command rendering to safely serialize multiline description values via escaped basic strings.
tests/test_extensions.py	Adds regression coverage to ensure multiline description values round-trip through tomllib.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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[RbBtSn0w]

#2096

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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[mnriem]

Thank you!