[route_check]: Add truncation indicator and mismatch summary counts

[Bojun-Feng]

What I did

Fix sonic-net/sonic-buildimage#27509

Added a truncation indicator and mismatch summary counts to route_check.py failure output.

When print_message() truncates output at PRINT_MSG_LEN_MAX (1000 chars), it now appends ... (truncated) within the length budget so operators know data was omitted. Additionally, a compact per-namespace mismatch count summary line is emitted before the detailed (potentially truncated) JSON dump.

How I did it

• Add PRINT_MSG_TRUNCATION_SUFFIX module-level constant and truncation marker logic in print_message() that keeps total length ≤ PRINT_MSG_LEN_MAX
• Extract summarize_results() helper to produce {namespace: {category: count}} dict
• Emit compact summary via json.dumps(..., separators=(",",":")) before the detailed failure results in both check_routes() and check_sids()

How to verify it

1. Run existing test_logging — confirms backward compatibility (all length assertions pass unchanged)
2. Run new test_logging_truncation_indicator — confirms truncation suffix behavior at realistic PRINT_MSG_LEN_MAX=50, short message passthrough, and edge case where max < suffix length
3. Manually: inject 200+ route mismatches on a VS image, run route_check.py -l INFO, verify summary count line appears and truncated output ends with ... (truncated)

Previous command output (if the output of a command-line utility has changed)

Failure results: {
    "": {
        "missed_ROUTE_TABLE_routes": [
            "2001:db8:1001::/64",
            "2001:db8:1002::/64",
            "2001:db8:10
Failed. Look at reported mismatches above

New command output (if the output of a command-line utility has changed)

Route mismatch counts: {"":{"missed_ROUTE_TABLE_routes":47,"missed_INTF_TABLE_entries":2}}
Failure results: {
    "": {
        "missed_ROUTE_TABLE_routes": [
            "2001:db8:1001::/64",
            "2001:db8:1002::/64",
            "2001:db8:10 ... (truncated)
Failed. Look at reported mismatches above

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