Security: path traversal hardening, mtime-keyed index cache, and ReDoS input limit

[purvi1508]

Summary

This PR addresses three issues identified in a security review of ⁠ clitable.py ⁠
and ⁠ parser.py ⁠, and adds a new standalone ⁠ cache.py ⁠ module.

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Changes

1. ⁠ textfsm/cache.py ⁠ — new file

A thread-safe ⁠ TemplateCache ⁠ class that stores compiled ⁠ TextFSM ⁠ instances
keyed by ⁠ (absolute_path, mtime_int) ⁠. When a template file is modified on disk
the stale entry is automatically evicted and recompiled on the next request.
Supports an optional ⁠ max_size ⁠ for LRU-style eviction and an ⁠ invalidate() ⁠
method for explicit flushing. A module-level ⁠ _DEFAULT_CACHE ⁠ singleton is
provided for future use by ⁠ CliTable ⁠.

2. ⁠ textfsm/clitable.py ⁠ — path traversal hardening + mtime cache

• New ⁠ _ValidatePath(path) ⁠ resolves any supplied path to its absolute form
  and checks it is contained within ⁠ template_dir ⁠. Paths that escape the root
  (e.g. ⁠ ../../etc/passwd ⁠) raise ⁠ CliTableError ⁠. The check is skipped when
  ⁠ template_dir ⁠ is ⁠ None ⁠, preserving backward compatibility.
• ⁠ _TemplateNamesToFiles ⁠ now calls ⁠ _ValidatePath ⁠ on every template name
  before opening, so a malicious entry in an index file cannot read arbitrary
  files.
• ⁠ ReadIndex ⁠ now keys ⁠ CliTable.INDEX ⁠ by ⁠ (absolute_path, mtime_int) ⁠ instead
  of a plain string. Stale entries for the same path are evicted automatically
  when the file changes on disk.

3. ⁠ textfsm/parser.py ⁠ — configurable input length limit

⁠ ParseText ⁠ and ⁠ ParseTextToDicts ⁠ each gain an optional ⁠ max_input_len ⁠
keyword argument (default ⁠ None ⁠, fully backward-compatible). When set, a
⁠ TextFSMError ⁠ is raised before any regex matching if the input exceeds the
limit, giving callers a straightforward way to guard against adversarially large
inputs that could cause catastrophic backtracking (ReDoS) in complex templates.

4. ⁠ tests/security_test.py ⁠ — new file

42 unit tests covering all three changes: cache hit/miss, mtime invalidation,
⁠ max_size ⁠ eviction, thread safety (20 concurrent goroutines), ⁠ _ValidatePath ⁠
with ⁠ .. ⁠ traversal and absolute escapes, traversal in template names,
⁠ ReadIndex ⁠ rejection, and ⁠ max_input_len ⁠ boundary values.