get_discussion and get_discussion_comments accept calls with missing required parameters instead of returning a validation error

[rodboev]

Describe the bug

When get_discussion or get_discussion_comments is called without one of the required parameters (owner, repo, or discussionNumber), the tool does not return a parameter-validation error. Instead it silently substitutes a zero value and issues a GraphQL request, which returns a confusing API-level error rather than a clear message identifying the missing field.

Affected version: current (tested against ghcr.io/github/github-mcp-server latest as of 2026-06)

Steps to reproduce

1. Configure the MCP server normally.
2. Call get_discussion with only {"repo": "myrepo", "discussionNumber": 1} (omitting owner).
3. Observe that the tool returns a GraphQL error like "Could not resolve to a Repository" rather than a message indicating owner is required.

Repeat with get_discussion_comments and any missing required field.

Expected behavior

The tool should return a structured error immediately: "missing required parameter: owner" (or equivalent), consistent with how add_discussion_comment and the other discussion write tools behave when a required parameter is absent.

Actual behavior

The tool issues a GraphQL query with the missing field set to its zero value ("" for strings, 0 for numbers) and returns whichever API error results from that invalid query.

Additional context

The write handlers in the same file (add_discussion_comment, reply_to_discussion_comment, etc.) were updated in PR #2718 to use explicit parameter validation that returns named errors on missing input. The two read handlers were not included in that pass and still use the legacy decoding pattern.