Support relative and absolute path keys in fileWindows lookup

[nordicnode]

Support relative and absolute path keys in fileWindows lookup

Summary

• In sdk/src/tools/read-files.ts, resolve fileWindows using filePath, relativePath, or fullPath.
• Previously, windows was looked up strictly via fileWindows?.[filePath]. If filePaths contained an alias (e.g. ./src/file.ts or an absolute path /cwd/src/file.ts) while fileWindows was keyed by the canonical relativePath (src/file.ts), or vice versa, the lookup evaluated to undefined.
• When windows is undefined while fileWindows is enabled, the code defaulted to [{}], dumping the whole file from line 1 and discarding the model's requested window offset/limit.
• Adding fallback lookups for fileWindows?.[relativePath] and fileWindows?.[fullPath] ensures requested windows are accurately applied regardless of path format.
• Adds regression tests in sdk/src/__tests__/read-files.test.ts verifying window resolution across relative, dot-slash, and absolute path variations.

Test plan

[✓] bun test src/__tests__/read-files.test.ts — 40 pass, 0 fail
[✓] bun run --cwd sdk typecheck — 0 errors
[✓] PR hygiene check passed