fix(deps): patch Dependabot security alerts

[abrichr]

Fixes all five open Dependabot alerts on uv.lock.

Alert	Package	Advisory	Severity	From	To
#12	anyio (transitive)	GHSA-82r6-8w77-94w6: TLSStream IDNA 2003 host name encoding	critical	4.12.1	4.14.2
#11	anyio (transitive)	GHSA-5p39-cfhj-2xmp: process-pool workers can block on undrained stderr	medium	4.12.1	4.14.2
#15	urllib3 (direct)	GHSA-8988-9cw3-xx77: HTTPS proxy TLS configuration may be ignored	high	2.7.0	2.8.0
#14	urllib3 (direct)	GHSA-vxq7-64xx-v4gw: unbounded chunk-size line buffered into memory	high	2.7.0	2.8.0
#13	urllib3 (direct)	GHSA-gh4c-6fx4-qh6g: chunked Deflate streaming can loop forever	medium	2.7.0	2.8.0

Changes

• uv.lock: only urllib3 and anyio change, each to its first patched release (uv lock -P urllib3==2.8.0 -P anyio==4.14.2). A plain --upgrade-package anyio selects 4.15.1, which also pulls a newer typing-extensions, so the lock names the exact versions.
• pyproject.toml: the urllib3 security floor goes from >=2.7.0 to >=2.8.0, so a later relock can't fall back.
• AnyIO comes in only through the dev and llm extras (httpx, anthropic). It gets no new declared dependency or override, because the lock upgrade is enough.
• tests/test_locked_security_floors.py (new): fails if the lock pins either package below its patched release, or if the urllib3 floor drops below 2.8.0.

No open Dependabot PR covered these alerts. The open Dependabot PRs (#224, #225, #226, #231) bump GitHub Actions only. This PR doesn't touch pyproject.toml or uv.lock lines that #232 changes.

Verification

• uv sync --locked --extra dev --extra llm installs urllib3 2.8.0 and anyio 4.14.2.
• uv run pytest tests/ -q: 519 passed, 1 skipped.
• uv run python scripts/validate_docs.py: passed.
• uv run python scripts/check_published_version_claims.py --offline: passed.
• uv run mkdocs build --strict: built.
• pip-audit --require-hashes on uv export --frozen --all-extras: no known vulnerabilities found.
• ruff check --target-version py312 and ruff format --check on the new test: clean. The repo has no ruff config or lint job.

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