fix: remove invalid boolean enum from issue_write delete parameter this appears in antigravity IDE cause error

[Mazen-Embaby]

This PR resolves the INVALID_ARGUMENT schema error by removing the Enum: []any{true} constraint on the boolean delete parameter in the issue_fields schema inside issue_write.

Since the Gemini API schemas only accept strings inside enum properties (repeated string enum in protobuf declarations), non-string values like the boolean true are serialized to an empty string "", triggering validation errors in client/bridge connections. Removing the redundant enum constraint fixes the issue and aligns the property definition with the one used in set_issue_fields.

Bug Description

When using the github-mcp-server with an LLM client (such as Gemini), the issue_write tool fails to load, returning a 400 INVALID_ARGUMENT error:

GenerateContentRequest.tools[51].function_declarations[0].parameters.properties[issue_fields].items.properties[delete].enum[0]: cannot be empty

Root Cause

The delete property within the issue_fields array in pkg/github/issues.go was defined with an enum constraint restricting it to a boolean true:

"delete": {
    Type: "boolean",
    Enum: []any{true},
    Description: "...",
}

Since the Gemini/client API schemas only accept strings inside the enum property (defined as repeated string enum in the protobuf declaration), non-string values like the boolean true are serialized to an empty string (""), triggering the validation error.

Solution
Remove the Enum: []any{true} constraint, making delete a simple optional boolean parameter, which is clean and matches the pattern already used in set_issue_fields (in pkg/github/issues_granular.go).