refactor: add strutil.InterfaceToIntString to deduplicate JSON numeric extraction

[Copilot]

Three packages independently inlined float64/json.Number → integer-string type-switches, with inconsistent safety: difc_log.go silently truncated non-integer floats (123.9 → "123"), while router.go validated but lacked json.Number support entirely.

Changes

• internal/strutil/numbers.go — new InterfaceToIntString(v interface{}) (string, bool) helper:
  • Explicit int64 range guard on float64 before conversion (avoids implementation-defined overflow)
  • Uses json.Number.Int64() to reject non-integer json.Number values
  • Returns ("", false) for all non-integer or out-of-range inputs

// float64(math.MaxInt64) rounds up, so >= catches the boundary overflow
if n < float64(math.MinInt64) || n >= float64(math.MaxInt64) {
    return "", false
}
i := int64(n)
if n != float64(i) {
    return "", false // non-integer float
}

• internal/server/difc_log.go — replaces 6-line switch with strutil.InterfaceToIntString; non-integer floats now return "" (bug fix, consistent with router.go)
• internal/proxy/router.go — replaces 12-line float64-only block with the helper, gaining json.Number support for free
• internal/strutil/numbers_test.go — new unit tests covering all numeric/non-numeric input types and edge cases
• internal/server/difc_log_test.go / internal/proxy/rest_backend_caller_tool_test.go — updated/added tests reflecting new behavior and json.Number coverage

[Copilot]

Pull request overview

This PR introduces a shared strutil.InterfaceToIntString helper to centralize “JSON-decoded number → integer string” extraction, replacing several ad-hoc type switches and aligning behavior across server logging and proxy routing while adding targeted unit tests.

Changes:

• Added strutil.InterfaceToIntString(v interface{}) (string, bool) to safely accept only in-range integer float64 and integer json.Number inputs.
• Refactored DIFC filtered-item logging and proxy routing to use the shared helper, eliminating duplicated numeric parsing logic.
• Added/updated unit tests to cover integer vs non-integer numeric cases and json.Number support.

Show a summary per file

File	Description
internal/strutil/numbers.go	Adds the shared numeric interface→int-string conversion helper with range/non-integer rejection.
internal/strutil/numbers_test.go	Introduces unit coverage for float64 and json.Number integer/non-integer and out-of-range cases.
internal/server/difc_log.go	Uses the helper to populate filtered-item number without truncating non-integer floats.
internal/server/difc_log_test.go	Updates expectations for decimal float handling (now rejected rather than truncated).
internal/proxy/router.go	Refactors route argument numeric extraction to use the helper (and should preserve prior validation/error semantics).
internal/proxy/rest_backend_caller_tool_test.go	Adds a json.Number test case to ensure numeric args are accepted beyond float64.

Copilot's findings

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• Files reviewed: 6/6 changed files
• Comments generated: 3

[lpcox]

@copilot address review feedback

[Copilot]

@copilot address review feedback

Addressed all three review items in commit fix: address review feedback on InterfaceToIntString and router validation:

1. numbers.go: json.Number case now captures the int64 from n.Int64() and returns fmt.Sprintf("%d", i) — canonical form that eliminates non-standard representations like "00123".
2. numbers_test.go: Updated the out-of-range float64 comment; added a leading-zero canonicalization test.
3. router.go: Restored specific error messages by type-switching on float64, json.Number before calling InterfaceToIntString. Invalid values (non-integer, out-of-range) now return "invalid %s (out of range or not an integer)", negatives return "invalid %s (must be non-negative)", and truly absent fields continue to return the "missing" error. New test cases cover all four invalid/negative combinations.