feat(ADDON-88124): add IndexName validator for Splunk index name validation

[okashaev-splunk]

Why

This PR is part of a multilayer change to replace current per-add-on length and regex index name validators with a central, authoritative validation endpoint. The new validation also covers MDL (Machine Data Lake) federated dataset names — ~.federated.<name> — which route data to Machine Data Lake via Ingest Processor and have naming rules that a single regex cannot handle correctly.

The changes:

• SplCore — implements POST /services/data/indexes/_validateName as the single point of truth for index name validation. All future rule changes happen here only, without requiring updates across individual add-ons.
• splunktaucclib (this PR) — adds IndexName validator that calls the SplCore endpoint. Falls back to a local reimplementation of the same rules to stay compatible with older Splunk versions where the endpoint is not yet available.
• splunk-add-on-ucc-framework — adds {"type": "index_name"} support to globalConfig.json schema and generates validator.IndexName() in REST handler code. Most add-ons have no custom REST handler and rely entirely on ucc-gen; without this change they would need to add a custom handler just to use the new validator.
• Technology add-ons — replace current length and regex validators in globalConfig.json with {"type": "index_name"}. Add-ons with existing custom validators replace those with IndexName directly.

What

Adds a new IndexName validator class to splunktaucclib/rest_handler/endpoint/validator.py.

How

• Calls POST /services/data/indexes/_validateName (Splunk core endpoint) to validate a candidate index name without creating anything.
• Session key is auto-detected from __main__.___sessionKey (written by MConfigHandler.__init__ from stdin before any handler method runs).
• splunkd URI is auto-detected from the SPLUNKD_URI environment variable, falling back to solnlib.splunkenv.get_splunkd_uri().
• Falls back to local pure-Python validation rules (mirroring IndexAdminHandler::validateName() from splcore develop branch) when the endpoint is unavailable: HTTP 404 (older Splunk versions), HTTP 400 with unsupported index_name argument, or no session key available.
• _call_validate_endpoint returns (None, None) to signal fallback, (True, None) for valid, (False, str) for invalid or request failure.
• validate_fallback and helpers (_validate_as_index_name, _validate_as_dataset_name) return a plain str — empty means valid, non-empty is the error message.
• Handles three name shapes: regular index names, federated: prefix (stripped before applying index rules), and ~.federated. MDL dataset names (routed to dataset-specific validation).
• ucclog logging throughout _call_validate_endpoint (WARNING for 404/400/403/5xx, ERROR for unexpected exceptions and malformed responses) and validate() (DEBUG for local fallback rejections).
• self._msg cleared at the start of validate() to prevent stale error messages when the validator instance is reused across requests.
• Lazy import of ucclog inside methods to avoid solnlib.log.Logs() being called at import time (requires Splunk runtime).
• Endpoint response parsing and HTTP error message extraction both wrapped defensively: except (KeyError, IndexError, TypeError, ValueError) covers missing keys, empty/null lists, null content, and non-JSON bodies.
• Zero-argument constructor — works as a drop-in RestField validator at class definition time, consistent with all existing validators.
• _INTERNAL_INDEXES frozenset sourced from splcore develop branch IndexAdminHandler::validateName().

Usage

RestField("index", required=True, validator=IndexName())

Tests

98 unit tests across five test classes:

• TestValidateFallback — parametrized valid/invalid name cases for local rules, covering regular names, federated: prefix, ~.federated. MDL dataset names, case-insensitive kvstore and default checks, length boundaries, and all reserved internal index names.
• TestGetSessionKey — reads __main__.___sessionKey; absent attribute returns None.
• TestGetSplunkdInfo — SPLUNKD_URI env var parsing; solnlib fallback when env var absent.
• TestCallValidateEndpoint — HTTP interaction: correct client.post() endpoint/body, correct SplunkRestClient constructor args, 404/403/400/5xx handling with log assertions, malformed response shapes (non-JSON, empty entry list, missing content key, content: null, messages: null, empty messages list), no session key path.
• TestValidate — orchestration: endpoint called with correct value, fallback triggered on (None, None), self.msg set correctly on failure, no-session-key falls back to local rules.