Fix binary resource blob encoding

[SamMorrowDrums]

Summary

Returns raw binary bytes through MCP resource blobs so the Go SDK performs exactly one base64 encoding. The same migration bug is fixed in both get_file_contents and repository resource reads.

Why

Fixes #3097.

Binary resources were manually base64-encoded before being assigned to mcp.ResourceContents.Blob []byte, causing encoding/json to encode the base64 text a second time. Images were corrupted after a spec-compliant single decode and could poison subsequent client conversation turns.

What changed

• Pass decoded file bytes directly to ResourceContents.Blob in get_file_contents.
• Pass raw response bytes directly to ResourceContents.Blob in repository resource reads, the only other affected production construction found by the audit.
• Marshal actual MCP results in regression tests, decode wire blobs once, and assert exact PNG/PDF bytes and PNG magic.
• Verify unencoded text content remains text while base64-encoded binary content follows the SDK-supported decode path.

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Binary resource behavior is corrected to the MCP wire contract. Tool names, descriptions, input schemas, and response shapes are unchanged.

Prompts tested (tool changes only)

• Fetch a PNG with get_file_contents and consume it as an image.
• Fetch a plain-text repository file with get_file_contents.
• Read an image through a repository resource URI.

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

The change removes the extra base64 layer and its persistent invalid payload growth; existing 1 MiB resource-link behavior and data exposure are unchanged.

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)

[Copilot]

Pull request overview

Fixes double-base64 encoding of binary MCP resources.

Changes:

• Pass raw binary bytes to MCP resource blobs.
• Add PNG/PDF wire-format regression coverage.
• Preserve text-resource behavior.

Show a summary per file

File	Description
pkg/github/repository_resource.go	Returns raw response bytes as blobs.
pkg/github/repository_resource_test.go	Verifies single-encoded PNG resources.
pkg/github/repositories.go	Removes manual blob encoding.
pkg/github/repositories_test.go	Tests text and binary wire serialization.

Review details

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• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Balanced