Respect image formatter settings

[Gabriel-Darbord]

Summary

• use a fresh image-configured formatter for each MCP method formatting pass
• stop overriding the formatter maximum line length
• remove the scoped-line-length test while retaining the method reformatting behavior test

Diagnosis

On Pharo 12 and 13, formatter instances share the default formatter context. Setting an instance maximum line length therefore changed the image-wide setting and made the test order-dependent.

Verification

• Pharo 12 focused reformatting test passes with the image maximum line length set to 70
• Pharo 12 full MCP-Tests and MCP-UI-Tests suite: 717 passed
• the image formatter maximum remains 70 after the full suite
• git diff --check passes