fix(discussions): validate required params in read handlers

[syf2211]

Summary

Validate required parameters in get_discussion and get_discussion_comments before issuing GraphQL queries, returning explicit missing required parameter errors instead of confusing API failures from zero-filled inputs.

Motivation

When callers omit owner, repo, or discussionNumber, the read handlers previously used mapstructure.WeakDecode, which silently substituted zero values and produced GraphQL errors like "Could not resolve to a Repository". The discussion write handlers were updated in #2718 to validate inputs explicitly; the read handlers were missed.

Fixes #2740

Changes

• Replace mapstructure.WeakDecode with RequiredParam / RequiredInt in GetDiscussion and GetDiscussionComments
• Remove unused mapstructure import from discussions.go
• Add regression tests for missing owner, repo, and discussionNumber on both tools
• Update comments in string-number tests to reflect RequiredInt behavior

Tests

go test ./pkg/github/ -run 'Test_GetDiscussion|Test_GetDiscussionComments' -count=1

Result: PASS

Notes

• String discussionNumber values (e.g. "1") continue to work via RequiredInt / toInt.
• Empty-string owner / repo now fail fast with the same missing-parameter message, matching write-handler behavior.