feat: add expect() and AssertableValue for generic fluent assertions

[othercodes]

Summary

Adds a generic fluent assertion layer (expect() / AssertableValue) so the same fluent vocabulary that powers AssertableJson, AssertableHtml, AssertableMCP, etc. extends to any value under test — domain objects, primitives, collections.

This is the foundational layer of pyssertive's mission: one fluent vocabulary for everything you test, from a single value to full HTTP responses.

from pyssertive import expect

expect(user.email).matches(r".+@.+").starts_with("alice")
expect([1, 2, 3]).has_count(3).contains(2)
expect(orders).each().has_attribute("status", "paid")

What's new

• expect(value) factory + AssertableValue class with ~30 matchers across:
  • Equality / identity (equals, does_not_equal, is_same_as, is_none, ...)
  • Truthiness (is_truthy, is_true, is_false, ...)
  • Types (is_instance_of, is_not_instance_of, is_type)
  • Comparisons (is_greater_than, is_at_least, is_between, ...)
  • Collections (has_count, contains, is_empty, ...)
  • Dict/object (has_key, has_attribute, ...)
  • Strings (matches, starts_with, ends_with, ...)
  • Higher-order (each(), sequence())
• Custom expectations via subclass — fully typed, IDE-autocompleted, mypy-checked.
• Top-level export: from pyssertive import expect, AssertableValue.

README improvements (also in this PR)

• New ### Expectations section at the top of ## Usage, framing expect() as the foundational fluent vocabulary.
• Tagline + first Features bullet updated to reflect "one vocabulary for everything you test".
• New top-level Table of Contents (752-line README needed it).
• MCP section refactored:
  • Big intro block split into 4 focused sub-sections (#### Initialize handshake / #### Tool call — success / #### Tool-level error / #### Protocol error).
  • New #### Method catalog with one sub-table per Assertable (AssertableMCP, AssertableToolList, AssertableToolDef, AssertableToolCall, AssertableContent).
  • MessageBuilder examples trimmed to 3 prototypical patterns + new flat method catalog table covering all 14 methods (including previously undocumented listing_resources / listing_prompts).
  • Orphan paragraph promoted to #### Catalog-wide invariants.
• ### HTTP — Basic Example merged into ### HTTP Status Assertions (no orphan section).
• > Requires the Django adapter note added to ### Database Assertions.
• Stale > Breaking change: assert_json() paragraph removed.

Test plan

• 99 new tests for AssertableValue covering happy + sad paths for every matcher.
• Parametrized tests for is_instance_of / is_not_instance_of covering single types and tuples.
• mypy strict clean (39 source files).
• Coverage 100% maintained.
• ruff lint + format clean.
• All 969 tests pass.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud