feat: support string login in build_config and add substitute_mapping_values (#63)

* feat: support string login in build_config and add substitute_mapping_values (#61, #62)

Extend build_config() to accept login: int | str | None. String logins
are coerced via the existing coerce_login() helper (empty/whitespace →
None, numeric strings → int, non-numeric → ValueError). When
allow_whole_dollar_env=True, ${ENV} and $ENV placeholders are expanded
before coercion, consistent with path/password/server behavior.

Add substitute_mapping_values(), a generic recursive helper that
substitutes environment placeholders in nested dicts/lists only for
caller-selected mapping keys. Non-selected fields (including literal
dollar signs) are preserved exactly. Supports blank_string_keys_as_none
to normalise empty strings to None after substitution. No application-
specific key names (e.g. mt5_login) are hard-coded in mt5cli.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Bump version to v0.9.5

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs+test: clarify substitute_mapping_values docstring and pin tuple behaviour

- Adds sentence noting list-element strings are never substituted (only
  immediate dict values are), addressing reviewer finding #1.
- Rewrites Returns section to accurately describe scalar pass-through
  behaviour, addressing reviewer finding #2.
- Adds recursion-depth caveat to the generic-utility docstring,
  addressing reviewer finding #4.
- Adds test_tuple_container_not_traversed to pin the existing silent
  tuple-exclusion contract, addressing reviewer finding #3.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: update public contract and README for build_config login coercion and substitute_mapping_values

- Expands build_config row to document login: int | str | None,
  numeric-string coercion, blank-string handling, and env placeholder
  expansion when allow_whole_dollar_env=True.
- Adds substitute_mapping_values to the stable SDK table with a note
  that key names are never hard-coded in mt5cli.
- Extends allow_whole_dollar_env paragraph to list substitute_mapping_values.
- README: adds build_config env-placeholder example and imports to the
  trading lifecycle snippet.
- README: extends credential-resolution bullet with a substitute_mapping_values
  usage example using generic key names.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: agent <agent@localhost>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "mt5cli"
-version = "0.9.4"
+version = "0.9.5"
 description = "Generic MT5 data and execution infrastructure for Python applications"
 authors = [{name = "dceoy", email = "dceoy@users.noreply.github.com"}]
 maintainers = [{name = "dceoy", email = "dceoy@users.noreply.github.com"}]