Store test case `time` as a number, not a string

[ubmarco]

Today

time is a string everywhere a need can see it:

• the field is declared a string in sphinxcontrib/test_reports/fields.py
  ("time": ("string", "Test execution time, in seconds")), the table both the
  extension's field registration and the converter's needs_schema read;
• the test-case directive parses the XML attribute to a float and writes
  str(...) back — sphinxcontrib/test_reports/directives/test_case.py:139-153,
  with the comment "SN 6.0.0 requires to be in one specific type and it is set
  to string for backwards compatibility";
• the converter mirrors the directive —
  sphinxcontrib/test_reports/needs_export.py, optional(case.get("time"), -1).

The parser already holds a float: junitparser.py:165,
float(testcase.attrib.get("time", -1)). The value is stringified only on its
way into a need.

So a needs.json says "time": "0.001", and nothing downstream can compare,
sum or average it without parsing it back. A needtable sorted by time sorts
lexicographically — "0.01" before "0.002".

Wanted

time declared as {"type": ["number", "null"]} and written as a JSON number:
"time": 0.001.

What has to change together

The declaration and both writers are one change and cannot move separately:
sphinx-needs type-checks a field's value against its declared type when the
need is created, and coerces only from a string to the declared type, never the
other way (needs_schema.py, convert_or_type_check). A number written against
a string declaration is an import-time error.

1. fields.py: "time": ("number", ...) — flows into both the extension's
   registration and the converter's needs_schema.
2. directives/test_case.py: pass the float, drop time_str.
3. needs_export.py: write the number instead of optional(...).
4. directives/test_suite.py and directives/test_file.py, which carry time
   on their needs too.

Migration notes

• An existing needs.json carrying "0.001" keeps importing: sphinx-needs
  coerces the string to the declared type.
• Filters and schemas in the field break — time == "0.001" and a
  {"type": "string"} entry in a needs_schema_definitions_from_json have to
  become numeric. Breaking change, worth its own changelog entry.
• Absent time needs one answer for both writers, which differ today: the
  parser reports -1, the directive writes "0.0", the converter writes "".
  With a nullable number, null is the honest value for "the report did not
  say", where 0.0 claims a measurement that never happened.
• case_line has the same shape of problem — declared a string, an integer in
  the XML. Same fix, separate decision.

Noticed while adding the needs_schema block to the converter's output
(#148), which is what makes the declared types visible in the artifact.