Add comprehensive test coverage for xarray backend engine and GridDAP functionality

[Divyateja2709]

This PR adds two critical test files to address significant gaps in test coverage:

tests/test_xarray_erddap.py - Complete test suite for the xarray backend engine
tests/test_griddap.py - Comprehensive tests for GridDAP protocol handling

Problem Statement
xarray Backend Engine (xarray_erddap.py)

Zero test coverage for a core feature
No validation of URL parsing and format detection
No testing of xarray plugin system integration
Missing error handling verification

GridDAP Functionality (core/griddap.py)

drawbacks:
Incomplete constraint validation testing
Missing edge cases for variable validation
No integration tests

Changes Made
File 1: tests/test_xarray_erddap.py
11 test classes covering:

URL Validation - HTTP/HTTPS detection, invalid URL rejection, edge cases
NetCDF Format Detection - Extension detection (.nc, .ncCF, .ncCFMA), query parameter handling
OPeNDAP Conversion - URL transformation, path preservation
Backend Entrypoint - xarray plugin registration, method validation
Dataset Opening - TableDAP/GridDAP support, multiple response formats, error handling
xarray Integration - engine='erddap' functionality, ERDDAP class integration
Data Integrity - CF attributes preservation, coordinate detection, metadata handling
xarray Operations - Selection/indexing, arithmetic operations, DataFrame conversion
Response Formats - .nc, .ncCF format handling
Error Handling - Invalid URLs, non-existent datasets, malformed requests
Edge Cases - Unusual patterns, empty datasets, boundary conditions

File 2: tests/test_griddap.py
7 test classes covering:

Constraint Fetching - Metadata retrieval, constraint structure, custom step sizes
Constraint Validation - User constraint validation, key detection, helpful errors
Variable Validation - Variable subset validation, invalid variable detection
GridDAP Initialization - griddap_initialize() method, step application, error handling
Download URL Generation - URL construction, subsetting, variable selection
Integration Testing - GridDAP to xarray conversion, end-to-end workflows
Edge Cases - Post-initialization modifications, unusual conditions

[ocefpaf]

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[1] https://github.com/ioos/gsoc/blob/main/contributor-guidance.md
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