Return client errors for invalid process and file paths

[chruffins]

Summary

• Accept unknown process IDs as strings so missing IDs reach handlers and return 404.
• Return 400 when GET /fs/list_files is given a file path.
• Add regression coverage for both cases.

Tests

• cd server && go test -race $(go list ./... | grep -v /e2e$) (the first run exposed the repository-generated SSE patch step was missing; after applying it, target suites passed)
• cd server && go test -race ./lib/oapi ./cmd/api/api
• cd server && go vet ./...

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Note

Medium Risk
OpenAPI breaking change for process path parameters (string without UUID validation) may affect API clients; process lookup behavior changes for malformed IDs but is limited to process management endpoints.

Overview
Process routes now treat process_id as a plain string in the OpenAPI spec and generated bindings (UUID format validation removed for status, stdout stream, stdin, and kill). Handlers look up IDs directly in the process map, so arbitrary unknown IDs (e.g. no-such-id) reach application logic and return 404 instead of failing at path parsing. Tests were updated accordingly, including broader 404 coverage for stdin and kill.

Filesystem listing maps syscall.ENOTDIR from os.ReadDir to 400 with "path is not a directory" when ListFiles is called on a file path, with a regression test in fs_test.go.

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