fix(docs): qualify catalog error severity note for get_product

[dylanfarrell]

Description

Fixes a misleading guidance note in the Catalog capability spec (docs/specification/catalog/index.md L173). The note claims all catalog errors use severity: "recoverable" and instructs agents to "retry, inform user, show alternatives", but the same spec tree shows get_product returning severity: "unrecoverable" for missing identifiers (rest.md:485, mcp.md:612), and the surrounding prose at rest.md:491–494 explicitly tells agents not to retry. The rewrite softens "All" to "Most", names the get_product exception, and links the two concrete examples. Prose-only change to one file; no schemas, transports, or other docs touched.

Category (Required)

Please select one or more categories that apply to this change.

• 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

None filed.

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. - NA
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas. - NA
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk. - NA

Screenshots / Logs (if applicable)

• uv run mkdocs build - built in 1.01s; the two new cross-links (rest.md#product-not-found, mcp.md#product-not-found) resolve to existing headings at rest.md:462 and mcp.md:586.